UsgsAstroFramePlugin refactor, part one (#117)

* Initial updates for refactoring the UsgsAstroFramePlugin

* Jacobian1 test now passing

* All coefficients test now passing

* More testing passing

* Down to 6 failing tests

* Fixed failing position-offset tests

* Update to strip out full-paths and also we're down to one failing test

* Fixed last failing test by updating canModelBeConstructedFromIsd to load the json ISD into a csm::Isd object if it has not already been loaded

* Updated based on review comments