feature request: user defined units

[shimwell]

There are some super useful units built into the plotter

However in some cases it might be handy if the user can specify the units.

My use case is when using an EnergyFunctionFilter the code can't be sure what units the function (with the current lack of units in tallies)

In this case it would be useful if the user can assign units to make use of on the colorbar

This could potentially be part of adding a scaling factor #105 as one could have a scaling factor units as well.