Refs #9448. Added PoldiSourceSpectrum class

A small wrapper class that extracts the wavelength distribution from the source component present in a POLDI instrument.
Refs #9448. Improved unit tests for PoldiSourceSpectrum
Merge branch 'feature/9448_representation_of_poldi_source_spectrum' into develop

Conflicts:
	Code/Mantid/Framework/SINQ/CMakeLists.txt
	Code/Mantid/Framework/SINQ/inc/MantidSINQ/PoldiUtilities/PoldiMockInstrumentHelpers.h