entities don't handle being set to np.inf

My data taking scripts ran into an edge case where it tried to set an entity to `np.inf`. The script just hung. I didn't even see an error in the service logs. I think the dripline should catch this case and try to intentionally crash or throw an error.

To go into more detail, here is the relevant portion of my control script.
```
popt_reflection, pcov_reflection = data_lorentzian_fit(s11_pow, freq, 'reflection')
perr_reflection = np.sqrt(np.diag(pcov_reflection))

print('Reflection lorentzian fitted parameters')
print(popt_reflection)
self.cmd_interface.set('f_reflection', popt_reflection[0])
self.cmd_interface.set('sig_f_reflection', perr_reflection[0])
self.cmd_interface.set('Q_reflection', popt_reflection[1])
self.cmd_interface.set('sig_Q_reflection', perr_reflection[1])
self.cmd_interface.set('dy_reflection', popt_reflection[2])
self.cmd_interface.set('sig_dy_reflection', perr_reflection[2])
self.cmd_interface.set('C_reflection', popt_reflection[3])
self.cmd_interface.set('sig_C_reflection', perr_reflection[3])
```
My script couldn't perform the fit on the VNA trace.
```
Setting na_measurement_status to start_measurement
Logging list of endpoints
Switching to transmission path
Switching to reflection path
Switching to transmission path
VNA reflection measurement
Setting na_measurement_status to start_measurement
Logging list of endpoints
Switching to transmission path
Transmission lorentzian fitted parameters
[1.61968618e+10 1.59075036e+04 2.62957230e-01 3.45835612e-03]
Switching to reflection path
/usr/local/lib/python3.7/site-packages/scipy/optimize/minpack.py:829: OptimizeWarning: Covariance of the parameters could not be estimated
category=OptimizeWarning)
Reflection lorentzian fitted parameters
[1.61903696e+10 4.84379319e+00 2.84176343e+02 2.84231409e+02]
```
I think when I get an OptimizeWarning error, the values of my `pcov` are `np.inf`, so it tried to set `sig_f_reflection` to `np.inf` and just hung without making any sort of progress.
```
➜ ~ kubectl logs -f double-precision-logger-dripline-python-deployment-6b7fbf8d4b7h --tail 20
{'timestamp': '2021-03-22T16:32:18.386845Z', 'sensor_name': 'Q_transmission', 'value_cal': 15907.503649437182, 'value_raw': 15907.503649437182}
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(49) -> finished processing data
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(46) -> insert data are:
{'timestamp': '2021-03-22T16:32:18.399052Z', 'sensor_name': 'sig_Q_transmission', 'value_cal': 570.3355496234211, 'value_raw': 570.3355496234211}
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(49) -> finished processing data
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(46) -> insert data are:
{'timestamp': '2021-03-22T16:32:18.411096Z', 'sensor_name': 'dy_transmission', 'value_cal': 0.26295723019139855, 'value_raw': 0.26295723019139855}
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(49) -> finished processing data
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(46) -> insert data are:
{'timestamp': '2021-03-22T16:32:18.423272Z', 'sensor_name': 'sig_dy_transmission', 'value_cal': 0.010107652156098478, 'value_raw': 0.010107652156098478}
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(49) -> finished processing data
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(46) -> insert data are:
{'timestamp': '2021-03-22T16:32:18.436062Z', 'sensor_name': 'C_transmission', 'value_cal': 0.003458356118361542, 'value_raw': 0.003458356118361542}
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(49) -> finished processing data
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(46) -> insert data are:
{'timestamp': '2021-03-22T16:32:18.449049Z', 'sensor_name': 'sig_C_transmission', 'value_cal': 0.00015618094767875254, 'value_raw': 0.00015618094767875254}
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(49) -> finished processing data
2021-03-22T16:32:26[INFO ] dripline.implementations.postgres_sensor_logger(46) -> insert data are:
{'timestamp': '2021-03-22T16:32:26.161902Z', 'sensor_name': 'f_reflection', 'value_cal': 16190369623.81609, 'value_raw': 16190369623.81609}
2021-03-22T16:32:26[INFO ] dripline.implementations.postgres_sensor_logger(49) -> finished processing data
2021-03-22T16:32:18[INFO ] dripline.implementations.postgres_sensor_logger(49) -> finished processing data
2021-03-22T16:32:26[INFO ] dripline.implementations.postgres_sensor_logger(46) -> insert data are:
{'timestamp': '2021-03-22T16:32:26.161902Z', 'sensor_name': 'f_reflection', 'value_cal': 16190369623.81609, 'value_raw': 16190369623.81609}
```