Severe memory leak in `populate_mock()`

Run the following test case and we observe the RSS size increase continuously. There is no apparent reason for this to happen reading from the face value of the code.

```
def test_hearin15():
"""
"""
model = PrebuiltHodModelFactory('hearin15')
try:
halocat = CachedHaloCatalog()
except:
halocat = FakeSim()
import resource
for i in range(100):
model.populate_mock(halocat)
maxrss = resource.getrusage(resource.RUSAGE_SELF).ru_maxrss
print(maxrss)

assert maxrss < 1000000
```

Result
```
../testenv/lib/python3.6/site-packages/halotools/empirical_models/composite_models/tests/test_preloaded_models.py 169816
185612
199336
216228
230180
230180
230180
241580
257364
276064
286356
291108
302724
315828
330084
345660
359916
...
```