Refs #9445. Removed using namespace from PoldiMockInstrumentHelpers.h

Had to adjust two unit tests to use the namespaces instead.
Refs #9445. Added clone method to PoldiPeakCollection
Refs #9445. Cleaned up PoldiSpectrumDomainFunction.
Refs #9445. Refactored PoldiCalculateSpectrum2D.

Most parts of the algorithm are tested by the unit tests now. The actual spectrum calculation will be done in a system test,
as instrument definitions etc. are required for this.