Cross platform command line driver + PropLib template adherence

[aromanielloNTIA]

• Adds a cross-platform command line driver as a utility for calling the P2108 software library. Inputs are stored in, and outputs are written to, a text file. The command line driver code resides in the app directory, is documented with Doxygen, and has its own README with further information: app/README.md
• Makes refactoring changes to avoid widespread use of preprocessor directives to hold constants (return codes, tags, and labels)
• Merges in changes from (NTIA-private) PropLib template repository, such that this repository is up to date with the template. This currently results in some minor and mostly non-functional updates to the files in this repository. In the future, this will allow for upstream changes to the template to be easily and cleanly applied to this repository.