DM-31783: Register the new N419 and N540 filters

[leeskelvin]

Original community PR: #199

There are two new narrow-/medium-band filters installed on DECam: an N419 narrow-band filter and an N540 medium-band filter. The N540 filter is used by the Merian survey, which depends on the science pipelines for data reduction.

• added the FilterDefinition of both filters to decamFilters.py.
• assigned the PS-1 g-band as the reference filter for both of them in config/calibrate.py, config/characterizeImage.py, and config/measureCoaddSources.py.
• added missing reference filter information for the N964 filter which has already been registered, assigning it to the z-band in the above three files.