Fisx

Provide access to default element properties.

Python 3.8 wheels and generated code.

Minor bug fixes

Python 3.8beta wheels

Fix clang problem related to compiler return value optimization.

Fix Python 3.7 build

Correct bug in getMultilayerFluorescence affecting the calculation of escape
peaks.

Implement a cache for calculation of escape peaks.

Introduce the "ratio" key in escape lines to provide the escape ratio.