Refs #9531. Created IFunction1DSpectrum interface.

IFunction1DSpectrum is an interface roughly equivalent to IFunction1D, with the exception that it must use FunctionDomain1DSpectrum.
Refs #9531. Integrated IFunction1DSpectrum into Fit and tests

Instead of having an extra property, fit now checks if the fit function can be cast to IFunction1DSpectrum to assign the correct domain creator.
Merge branch 'feature/9531_SpectrumSeqDomainCreator' into develop