Mwalib

* Added alternative version of mwalib_metafits_context_new (mwalib_metafits_context_new2) to FFI interface which does not require an MWAVersion and will determine it via the MODE keyword.
* Fixed errors, ommissions in comment/documentation for FFI function mwalib_metafits_get_expected_volt_filename().

* Added mwa_version <Option<MWAVersion>> to MetafitsContext struct.
* When working only with a MetafitsContext, a None can be passed in lieu of an MWAVersion, and mwalib will attempt to determine the correct MWAVersion based on the MODE keyword from the metafits file.
* Added method get_expected_volt_filename() function to MetafitsContext.
* Added digital_gains, dipole_gains and dipole_delays to FFI rfinput struct.
* Added num_receivers, num_delays to MetafitsContext / metafits_metadata struct in FFI.
* Metafits metadata sched_start_utc and sched_end_utc are now typed as time_t and represent the Unix timestamp (which can be used by various time.h functions).

* Updated some dependencies in Cargo.toml.
* Renamed metafits_context.metafits_fine_chan_freqs by adding _hz suffix to be consistent with other naming of attributes with units.

* Fixed bug where the metafits.num_metafits_fine_chan_freqs was not being set correctly.

* Added helper function get_fine_chan_freqs_hz_array to correlator context and voltage context.
* Added metafits_context.num_metafits_fine_chan_freqs & metafits_context.metafits_fine_chan_freqs, providing a vector of sky frequencies for all fine channels.
* Added metafits_context.volt_fine_chan_width_hz & metafits_context.num_volt_fine_chans_per_coarse to describe the voltage fine channel configuration.
* Added the above new functions and attributes to equivalent structs in FFI.
* Added more badges to github README.

* mwalib legacy autocorrelations (where ant1==ant2) are now conjugated with respect to previous versions.