Import package gamma_simp by Barton Willis.

Import package gamma_simp by Barton Willis.

Copied verbatim as retrieved by `git clone https://github.com/barton-willis/gamma_product_simp.git` today,
with only the following exceptions:

 * Top-level folder is gamma_simp instead of gamma_product_simp,
   to make the folder name coincide with the name of the file that gets loaded.

 * Change line endings from CRLF to LF in gamma_simp.mac and rtest_gamma_simp.mac
   to make them the same as all other .mac files in share.

 * Remove .gitattributes which had `* text=auto` which caused unrevertable
   line ending changes.

Thanks to Barton for this contribution.