rawspec ignores AZ and ZA GUPPI RAW headers

[david-macmahon]

rawspec ignores the AZ and ZA GUPPI RAW headers. These values should be read from the first RAW file of a sequence (i.e. the *.0000.raw file) and used to populate fields (not yet present) in rawspec_raw_hdr_t which should then be used to populate the az_start and za_start fields of a fb_hdr_t instance. Because these fields are currently ignored, the az_start and za_start fields in the output files end up being 0.0, which is their default value.

Ideally, these fields in fb_hdr_t should get initialized with nan() and then only included in the filter bank header if they are non-NaN.