Keithley 2700: Ordering issue with clearing buffer [TIMEBOX: 1 day and a half]

As a developer I would like to be sure the Keithley 2700 correctly reads from the device after clearing the buffer. Currently you can see from the emulator that the IOC does the following:
1. Asks the next location on buffer
2. Clears the buffer
3. Asks the next location on buffer
4. Reads from the buffer assuming value in 1) rather than in 3)

```
021-03-09 17:16:25,197 INFO lewis.Keithley2700StreamInterface.StreamHandler: Processing request b'TRAC:NEXT?' using command TRAC:NEXT\?$
2021-03-09 17:16:25,197 INFO lewis.Keithley2700StreamInterface: Next buffer location: 16
2021-03-09 17:16:25,199 INFO lewis.Keithley2700StreamInterface.StreamHandler: Processing request b'TRAC:DATA:SEL? 14,2' using command TRAC:DATA:SEL\?\ ([+-]?\d+),([+-]?\d+)$
2021-03-09 17:16:25,199 INFO lewis.Keithley2700StreamInterface: Returned readings: +1084,6.164654970169067,+102,+1400,6.265652656555176,+103
2021-03-09 17:16:25,227 INFO lewis.SimulatedKeithley2700: Inserting mock data into buffer: ['+1583,6.367274522781372,+104']
2021-03-09 17:16:25,266 INFO lewis.SimulatedKeithley2700: Setting simulate readings to False
2021-03-09 17:16:25,386 INFO lewis.Keithley2700StreamInterface.StreamHandler: Processing request b'TRAC:CLE' using command TRAC:CLE$
2021-03-09 17:16:25,386 INFO lewis.SimulatedKeithley2700: === Cleared Buffer ===
2021-03-09 17:16:25,394 INFO lewis.Keithley2700StreamInterface.StreamHandler: Processing request b'TRAC:NEXT?' using command TRAC:NEXT\?$
2021-03-09 17:16:25,394 INFO lewis.Keithley2700StreamInterface: Next buffer location: 0
2021-03-09 17:16:25,395 INFO lewis.Keithley2700StreamInterface.StreamHandler: Processing request b'TRAC:DATA:SEL? 16,984' using command TRAC:DATA:SEL\?\ ([+-]?\d+),([+-]?\d+)$
2021-03-09 17:16:25,395 ERROR lewis.Keithley2700StreamInterface: An error occurred at requestb'TRAC:DATA:SEL? 16,984': IndexError('list index out of range')
2021-03-09 17:16:25,396 ERROR lewis.Keithley2700StreamInterface: Traceback (most recent call last):
File "C:\Instrument\Apps\Python3\lib\site-packages\lewis\adapters\stream.py", line 122, in found_terminator
reply = cmd.process_request(request)
File "C:\Instrument\Apps\Python3\lib\site-packages\lewis\adapters\stream.py", line 398, in process_request
return self.map_return_value(self.func(*args))
File "C:\Instrument\Apps\epics\support\DeviceEmulator\master\lewis_emulators\keithley_2700\interfaces\stream_interface.py", line 238, in get_readings
chunks.append("{},{},{}".format(self._device.buffer[buffer_location].reading,
IndexError: list index out of range
```

## Acceptance Criteria
* Fix the IOC to use the correct buffer length value
* Write tests to catch this
* Confirm that it still works on actual device