Bug in `applyInputParams` of multiple material classes

The applyInputParams methods of multiple material classes contain a common bug when applying enrichments to particular isotopes/elements. Take, for example, adjusting the enrichment of B10 within the B4C class:

 def applyInputParams(
        self, B10_wt_frac=None, theoretical_density=None, *args, **kwargs
    ):
        if B10_wt_frac:
            # we can't just use the generic enrichment adjustment here because the
            # carbon has to change with enrich.
            self.adjustMassEnrichment(B10_wt_frac)

If the user intentionally specifies B10_wt_frac=0, as in they want their absorber to have only B11, the enrichment will not be adjusted because the conditional evaluates to False.

This same bug applies to other material classes, including UO2, and possibly others.

[john-science]

PR Merged! Thanks @keckler!

[keckler]

PR added at #425