No way to export source_rate units when writing step in depletion simulation

[lewisgross1296]

Description

Add units to the source_rate data exported to h5 on depletion step writes. Currently, restart or continue runs (coming soon) just assume the user has the same units in the new simulation as the initial. The simulation will run and not error if you change between power, power_density, or source_rate when providing new data to the Operator (or reading from a Results file).

Alternatives

Burden the user to ensure they use the correct units when restarting/continuing depletion.

Compatibility

Probably don't not need to change the API but would add some checking / verification on the back end when going to/from hdf5.

See #3272. Tagging @pshriwise in case I missed something important