Use .dat fixed width format
Use .dat fixed width format

Why:

The Amber .dat format has a fixed width specification.
The previous implementation simply splits by column, which
breaks when it encounters lines such as :

c1-sh-hs   48.23       95.99	calculated_based_on_C#C-SH       0

This change addresses the need by:

* Replacing the .dat parsing code in AmberParser with functions
that parse lines according to the fixed-width format.
* Explicitly naming the variables according to the Amber format,
so it is clearer from the code what is being stored.

choderalab/openmoltools,choderalab/openmoltools