Dies if filtering empty hit table

[Adamtaranto]

Throws error and dies when attempting to filter hits on prop model coverages if pandas table contains no hits.

Fix: Skip all downstream steps if no hits are imported OR if no hits remain after a filtering step.

Example:

Log: Loading nhmmer hits from: TIR32bpuniquealn.tab 
Log: Imported 0 hits from 1 models. 
Log: Filtering hits with < 0.4 model coverage. 
/Users/adamtaranto/anaconda/envs/py3/lib/python3.6/site-packages/pandas/core/ops.py:798: FutureWarning: elementwise comparison failed; returning scalar instead, but in the future will perform elementwise comparison
  result = getattr(x, name)(y)
Traceback (most recent call last):
  File "/Users/adamtaranto/anaconda/envs/py3/bin/tirmite", line 11, in <module>
    load_entry_point('tirmite', 'console_scripts', 'tirmite')()
  File "/Users/adamtaranto/ScienceProjects/TE-Projects/TIRmite/tirmite/cmd_tirmite.py", line 190, in main
    hitTable = tirmite.filterHitsLen(hmmDB=hmmDB, mincov=args.mincov, hitTable=hitTable)
  File "/Users/adamtaranto/tirmite/__init__.py", line 417, in filterHitsLen
    **hitTable = hitTable.ix[~((hitTable['model'] == model) & ((hitTable['hitEnd'].astype(int) - hitTable['hitStart'].astype(int)) + 1 < minlen))]**
  File "/Users/adamtaranto/anaconda/envs/py3/lib/python3.6/site-packages/pandas/core/ops.py", line 861, in wrapper
    res = na_op(values, other)
  File "/Users/adamtaranto/anaconda/envs/py3/lib/python3.6/site-packages/pandas/core/ops.py", line 800, in na_op
    raise TypeError("invalid type comparison")
TypeError: invalid type comparison