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.
Merge branch 'feature/9990_poldi_2d_background_functions' into develop