MNT : PEP8 white-space fixes
MNT : re-factored back end data structures

 - turned OTHER_VAL into a dict keyed on both
   the symbol and Z
 - simplified the data look up in Element
MNT : simplified handling of energy

  - just try to cast to float.  Let python deal with the conversion.
    If it can't be done it will raise an exception.
MNT : use np.nan for missing values

I am interpreting 0 in the density of Fm as meaning 'not enough of this
has ever been made'.  Use np.nan so that computations using this value
will fail (yield nan), rather that silently, but incorrectly, work.
ENH : make any cases work for element symbols

force all string inputs to be lowercase when keying the
dictionary.
Merge pull request #2 from tacaswell/xrf_data

Xrf data