Refs #8849. Move common validating functionality to a method.
Refs #8849. Default values as strings.

Plus a single time-zero default value.
Refs #8849. Move all the Xmin/Xmax setting logic to functions.
Refs #8849. Move option tab validation to the tab code.
Refs #8849. Constants for default values which are used in two places.
Refs #8849. Validation for Y min/max fields.
Refs #8849. Change the type of functions back for consistency.
Refs #8849. Move rebin parsing to options tab code and add validation.
Refs #8849. Make some fields pass validation when empty.

We have some fields which are allowed to be empty, which means they will
be calculated automatically. Make validator pass when they are empty.
Refs #8849. Add missing Doxygen @param description
Refs #8849. Merge remote-tracking branch 'origin/master' into 8849

Conflicts:
	Code/Mantid/MantidQt/CustomInterfaces/inc/MantidQtCustomInterfaces/MuonAnalysisOptionTab.h
	Code/Mantid/MantidQt/CustomInterfaces/src/MuonAnalysisOptionTab.cpp
Merge remote-tracking branch 'origin/feature/8849_muon_analysis_validators'