Added GXP support to read_gpf() (#164)

* Added GXP support to read_gpf()
Modified read_gpf() in io_bae.py to support reading of either legacy Socet Set-style GPF files or new Socet GXP-style GPF files. Function now takes a boolean, `gxp` to flag whether function should interpret input file as SSET or GXP style, defaulst to SSET style for backwards compatibility. Removed unused variables from read_gpf(). Field names are now hard-coded in the function, rather than generated dynamically. The hints about field names in the GXP-style GPFs are just complicated enough that hard-coding a list of field names into the function was the simplest solution. Datatypes for each field were already hard-coded, so hard-coding the names is stylistically consistent.

Partially address #24

* Deleted extraneous comment

* Added tests for read_gpf() and updated Socet GXP test data

* Updating test for read_gpf(), ticking minor version number

* Fixing fixtures...maybe

* Fixing fixtures...maybe

* Correct variable assignment in read_gpf(), more fixing fixtures

* Fixing test parameterization

* More test parameterization tweaks