Allow normalization postprocessors to be defined by user

[aprilnovak]

Reason

Within NekRSProblemBase, we currently add a few postprocessors (flux_integral, source_integral) to receive power values from some coupled application to renormalize things inside NekRS. These postprocessors are set to Receiver values. However, in some cases the user may want to do more complicated behavior (like setting flux_integral to the sum of two other postprocessors). However, the current configuration precludes the user from doing so (they will get an error along the lines of duplicate postprocessors being added by the same name).

Design

Allow flux_integral and source_integral to be defined by the user.

Impact

New feature, necessary to support coupled radiation + conductive fluxes in NekRS.

fyi @regerdavid