add option for emissivity weighting

[fgroth]

add option for emissivity weighting for grid assignment and filtering.

[fgroth]

I did the following checks on the new implementation:

• The code compiles with any combination of the new options
• It runs with any combination of the new options
• The filtering scale looks reasonable compared to other weighting modes.
• The filtered velocity appears reasonable, looking at a slice through one simulation.