Refs #9990. Added linear background function for POLDI

A function that is linear in workspace index, as a background function for POLDI 2D-fits.
Refs #9990. Behavior of Poldi2DFunction now match CompositeFunction
Refs #9990. PoldiCalculateSpectrum2D now has background terms

Background functions are added if specified by the user.
Refs #9990. Cleaning up.
Refs #9990. Removing debug output.
Refs #9990. Added algorithm properties to make testing easier.
Refs #9990. Added background example in documentation.
Refs #9990. Fixing unit test of Poldi2DFunction

Changed the behavior of PoldiSpectrumDomainFunction to be usable with CompositeFunction, had to adjust the unit test here to reflect that change.
Refs #9990. Fixing unit test of PoldiCalculateSpectrum2D

Forgot to include the composite function modifier in the tested string.
Merge remote-tracking branch 'origin/feature/9990_poldi_2d_background_functions'