Refs #10623. Added method to convert intensities back to counts

Also fixed a problem in PoldiMockInstrumentHelpers, where 2 d-values where wrong.
Refs #10623. Added properties to algorithm.

The algorithm now also produces a PoldiPeakCollection with the fitted peaks.
Refs #10623. SeqDomainSpectrumCreator takes masking into account

When the detector corresponding to a spectrum of the matrix workspace is masked, it's not added to the domain.
Refs #10623. Fixing faulty weights in FunctionDomain1DSpectrumCreator

Had to adjust the errors on the tests a bit, because the weights were wrong before.
Refs #10623. Changed Poldi2DFunction to adjust weighting scheme.
Refs #10623. Renamed algorithm to PoldiFitPeaks2D

A separate calculation algorithm is not required, so the algorithm is renamed to better reflect what it's doing.
Refs #10623. Adding usage example for PoldiAnalyseResiduals
Refs #10623. Preserving HKLs through fitting process.
Refs #10623. Adding header for sqrt.
Merge branch 'feature/10623_enable_poldi_2d_fitting' into develop