Add Doc strings from HSPF descriptions

[aufdenkampe]

HSP2 is presently functionally identical to HSPF, with identical names for modules, sections, routines, and variables, and with identical water simulation algorithms. This provides both credibility and also access to the excellent HSPF documentation.

A goal for HSP2 is coupling with other models, which would be facilitated by adding detailed documentation strings to the code base, including adding long name aliases for the current <8 character Fortran names. In addition, HSP2 can be run from a variety of new interfaces not previously available to HSPF users.

The long term goal is to develop a complete suite of HSP2 manuals and tutorials through automated document generation using Sphinx or a similar document generation library and through tutorials and example code implemented in interactive Jupyter Notebooks.

This task is a first step toward those documentation goals, by copying long names and descriptions from the HSPF manual into HSP2 doc strings.