Added ZAID function to convert from ZAID to (Z,A)
Added Nuclide class, which has flexible input
Added __repr__ to Nuclide class
Enable module to be imported from another dir

When importing from another directory, the text data files
read by nuclide_data.py where not found. Added basepath to
correct this problem.
Added metastable parsing

To accept strings like "Am-242m" and "AM242M"
Comparison operators for Nuclide class
Added weight attribute to Nuclide

I need to resolve missing weights for common
metastable nuclides.
Added some metastable energies

To enable weights for these nuclides
Added a few more metastable nuclide energies
Added decay_const and half_life member to Nuclide
Added a few more metastable isotopes
Added MAT id with ENDF list text file

Now I have can lookup ENDF MAT number with Nuclide class.
Fixed divide-by-zero warning and Nuclide(E, metastable) logic

If metastable=True was specified without E, then sometimes
metastable wouldn't actually get set. It should be fixed now.