Fix compilation bug in xectprec.h; remove envmod stuff

Remove creation of envmod files from the Makefile.am for the overall project.

xextprec.h was having a weird compile error; so I've removed the function causing the problem for now (since xextprec is still in development)

Fix Gpc

The Gpc (k_dGpc) was 10^12 pc instead of 10^9 pc.

Add earth and jupiter mass and radius; fix AMU mass

This version adds the mass and radius of Jupiter and Earth. It also corrects the conversion from AMU to grams (was off by a factor of 1000).

Updated constants in xastro

This release updates the constants in xastro to those defined by the IAU in 2012 and 2015 as well as the new definitions of the fundamental charge and other rearrangement of fundamental constants in CODATA 2018.

v1.8.0: xstdlib::command_line_parse, erg<->cm^{-1} (xastro)

Add c++ command line parser to xstdlib. Has functionality similar to the older C command line parsing routines, but parses the entire command line once and saves the values, and also can track allowed options and types, and return parameters with incorrect types or unrecognized keys

Add conversion from erg to/from cm^{-1} in the xastro constants.

fix to xio::xdataparse

xdataparse would hang when parsing a line which had a empty element at the end (e.g. 1,2,3,4,). This update correctly ends the parsing and inserts an empty element at the end.

xio and xstdlib additions

Add functions to xdataset_improved to zero the table, and read xdataset files.
Add function to xstdlib to read command line paramters, returning the result using std::string
Add function to xstdlib to determine the atomic number based on an atomic symbol

xlinfit; xexpfit; Rydbergs to cm^-1

Add the xlinfit and xexpfit tools for fitting a line or exponential to a dataset; xlinfit has been migrated from snatk.
Add conversion from Rydberg to cm^-1 in xastro.

xastro and xmath updates

xastro constants are now all internally consistent -- those that derive from fundamental constants are calculated based on those constants rather than being hard-coded.

Add rotation matrix loading capability to xsquare_matrix_long with ability to specify axis of rotation (only unit axes are allowed); add ability to load derivatives of rotation matrices as well.

1.2 and 1.3 are internal releases.

shared libs, dereddening and redshift

Switch to using libtools for building the libraries for broader platform support.
Add to xastro spectral dereddening routines based on Cardelli, Clayton, & Mathis (1989) with O'Donnell (1992) correction to optical.
Add to xastro routines for adjusting wavelength based on redshift.