Implement median filter to photometer method

The photometer method computes synthetic broad-band fluxes from PFS spectra. For objects with relatively low-S/N, their fluxes suffer from artifacts in the spectra such as cosmic ray or sky residuals. A simple median filter should be able to get rid of (most of) them and I suggest to implement the option to the photometer method. The size of the wavelength window to compute median is a config parameter, but I have been using +/-0.5nm (i.e., 1nm width, which is about 10-15 pixels) in my code and it seems to be working fine.

As the broad-band photometry does not require a high spectral resolution, we could simply bin PFS spectra into 1nm grids and take the median in each grid. The compute time will be much shorter than applying the running median filter at each wavelength point.