Add support for user-defined spectral weighting in multi-frequency matched filter

[mattyowl]

Currently, the only spectral weighting implemented in nemo is for the SZ. The user should be able to define any spectral weighting (e.g., using a power law). This would have to go in the 'params' key for filter(s) defined in the config file. This could then be used to construct MMF catalogs for dusty sources, AGNs, etc., as in e.g. Gralla et al..