Fisx

Implement a calculation cache.

Implement a configurable threshold on emission rate of an x-ray line to
consider secondary excitation originated from that emission line.

Correct problem calculating partial photoelectric cross sections in the
vicinity of absoprtion edges.

Correct inconsistency on M-shell data

Correct problem calculating partial photoelectric cross-sections close to the
Mo edge.

Correct error on internal method name (issue 18)

Forced release to comply with PyPI.

Allow to set a data directory from the environemnt.

Get rid of unused code.

Adapt for PyMca-like initialization with PyMca versions relying nn fisx data.

Add source of the data to the data files.

Specific additions for Debian testing during build.