Mwalib

* Provide DUT1 in the metafits context
* API tweaks surrounding generic types
* Things should be slightly more flexible than before
* Clippy lints and update cbindgen
* Rename mwa_rust_core -> Marlu

* Specify minimum-required dependency versions.
* Clean up fits long-string functions.
* Stop borrowing generic types.
* This applied only to single files (like a metafits file); the required was
type `&T` (where `T` is `AsRef<Path>`) when `T` alone is sufficient.
* Clippy lints and Rust 2021.
* Appeased most Clippy lints and upped the Rust edition to 2021. mwalib now
also specifies the minimum supported Rust version and has CI tests against
this version.
* Lighten code to get channel frequencies.
* Changed metafits_context `cable_delays_applied` from bool to an enum `CableDelaysApplied`.

* Fixed calculation of number of baselines for cases where num_antennas is odd.

* rfinput->digital_gains are now f64 and are divided by 64 (the value in the metafits is 64x larger to allow more flexible gains settings)
* Modernised examples using clap 3.0 and reduced dependencies

* `get_optional_fits_key` now handles the CFITSIO return code of 204 (when there no value for the key) and correctly returns None instead of an error
* The `digital_gains array` in `rf_input` is now sized based on the `num_metafits_coarse_chans` rather than hardcoded to be 24
* `VoltageContext`'s voltage batches and voltage time map are now (library) public like the equivalent attributes in `CorrelatorContext`
* Updated code coverage github action as it had stopped working

* Fixed issue and covered with tests the case where mwalib would panic when creating `CorrelatorContext` with only gpubox file(s) from the second or higher batch (e.g. 001,002...).