Save antialiasing filter coefficients to file

Many metadata are saved to file along with the main measurements, including dt, t0, size, physics upsampling, orbits used, etc. However, the antialiasing coefficients are not saved. These are needed for the on-ground processing, as we might want to know the filter transfer function or the filter group delay applied on the data (function of the filter order, ie, number of coefficients).

We should save aafilter_coeffs as attribute to HDF5 file.