Mwalib

* Refactor of FFI: collapsing, antennas, baselines, coarse channels, rf inputs and timesteps into attributes of the MetafitsMetadata, CorrelatorMetadata and Voltage Metadata structs.
* Refactor of FFI: Added common, common good and provided timesteps/coarse channels to CorrelatorMetadata and VoltageMetadata structs.
* Refactor of FFI: Added delays and recievers to MetafitsMetadata.
* Added const for speed of light in a vacuum.
* Detected and raise error condition when gpubox fits file has no data.
* Updated VoltageContext display to be consistent with CorrelatorContext.
* Fixed bugs when reading metafits files which have new MWAX GEODEL, CALIBDEL and CABLEDEL keys.

* Added common, common good and provided timesteps/coarse channels for VoltageContext.
* Voltage timesteps and coarse channel vectors now represent the superset of metafits and provided data.
* Due to the above, the read methods now may return a NoDataForTimeStepCoarseChannel error if the timestep/coarse channel combination does not have a file available to read data from.

* Modified logic of common and good correlator timesteps/coarse channels to mean common to all provided coarse channels.
* Added provided_coarse_chans which is a vector of all of the provided gpubox files coarse channels.

* Removed VisibilityPol struct, replaced with VisPol enum to simplify.
* Renamed CorrelatorVersion enum to MWAVersion. This now incorporates Correlator version (OldLegacy, Legacy and MWAXv2, as well as VCSRecombined and MWAXVCS).
* MetafitsContext struct now requires an extra parameter: mwa_version in order to correctlty interpret the metadata when only a metafits file is supplied (and no data files).
* Renamed COAX_V_FACTOR to MWA_COAX_V_FACTOR to bring it in line with other mwalib consts.
* Added metafits_coarse_chans to MetafitsContext which represent all the coarse channels that the metafits file defines.
* Added metafits_timesteps to MetafitsContext which represent all the timesteps that the metafits file defines.
* Added geometric_delays_applied enum to MetafitsContext & FFI to inform the caller if and what type of geometric delays have been applied to the data.
* Added cable_delays_applied bool to MetafitsContext & FFI to inform the caller if cable length corrections have been made to the data.
* Added calibration_delays_and_gains_applied bool to MetafitsContext & FFI to inform the caller if calibration delays and gains have been applied to the data.
* Added mode enum to MetafitsContext & FFI to inform the caller of the mode of the observation.
* Added electrical_length_m, north_m, east_m, height_m to the Antenna struct (for efficiency). These are also available in the Rfinput struct.
* Added common timesteps and coarse channels which represent the timesteps and channels common the most max set of coarse channels provided.
* Added common good timesteps and coarse channels which is the same as common, except only timesteps after the quack time are used to determine them.

* Implemented _into_buffer variant of read_by_baseline and read_by_frequency so you can allocate your own vector and pass the whole thing or a slice to be filled with the relevant data.
* The implementation of the ffi read_by_baseline and read_by_freqeuency now use this method, thus reducing an unneeded allocation and copy.
* Added built crate to allow rust callers to query the mwalib build environment (including the mwalib version number).
* Exposed major, minor and patch version of mwalib to ffi callers.
* Pinned rust nightly build in coverage CI to prevent a missing crate error (temporarily?)

* Added support for reading voltage data by file or gps second.
* Added FFI support for reading voltage data by file or gps second.
* Removed num_samples_per_timestep from VoltageContext. Added more useful struct members describing the data shape precisely.
* Removed unneeded muts from correlator and voltage contexts.
* Minor cleanup of rust examples.