lib/igc: Parse HFDTE line from IGC files
lib/igc: Don't break on missing logger information

Some old Volkslogger firmware produced valid files that are missing
the first A line
model/igcfile: Save HFDTE header as date_utc
model/igcfile: Added NOT NULL flag to date_utc column

/flights/igc_headers should be called before applying this patch to fill
the empty date_utc column
lib/igc: Moved filename_to_path() call to caller

This allows us to unittest the read_igc_headers() method by passing a list
of strings
tests/lib: Added unittest for skylines.lib.igc