DOC : fixed numpydoc + autosummary + classes

Turns out numpy uses a custom template for formatting the autosummarry
output for classes, use this template as well.

Make sure conf.py re-generates all of the auto-summary created files.
DOC : edits to documentation

Edited Element and XrayLibWrap documentation to play nicer with
the new template
DOC : more work on improving the constants.py docs

 - tweaked elemental documenation
 - documented namedtuple helper classes
 - documented calibration standard dictionary
DOC : fix typo
ENH : made changes to doc/length calculation
TST : added more tests
Merge pull request #146 from tacaswell/doc_work

Doc work