Allow non-standard head3.lst path in ms_analysis.py

[CatChenal]

Tests:

PREREQUISITE: A conda environment for mcce exists and is activated.

1. Running test_ms_analysis.py in a mcce output folder:

   $ test_ms_analysis.py head3.lst
   >>> len(msa.conformers) = 1673
   >>> len(new_conformers) = 1673

2. Running test_ms_analysis.py outside a mcce output folder:

   $ cd ~/
   $ test_ms_analysis.py projects/mcce_data/head3.lst
   >>> len(msa.conformers) = 0
   >>> len(new_conformers) = 1673