Implement generic way to handle Debye-Waller factors in interference functions

Remove older presences of position variance and provide translations of the parameter to Python and GUI

Fix possible numerical instability in InterferenceFunctionRadialParaCrystal

Fix memory leakages in PythonAPI

Refactoring of CsvImportTable

1. CsvMultiplierField does not add new functionality to ScientificSpinBox,
   thus it can be replaced with the parent class.
2. Since the enitity is used only in one compilation unit, it can be
   placed into an anonymous namespace in a cpp file.

Redmine issue 2259: restore off-spec fitting functionality

Redmine: #2259

Redmine: #2259