Sdss-lvmscp

✨ Improved

* Added `V_LVMSCP` header keyword with `lvmscp` version.
* Added `SKYENAME` and `SKYWNAME` to header model.
* Added `POSCIPA`, `POSKYEPA`, and `POSKYWPA` to header model.

✨ Improved

* Added default header keywords for guider and standards.

✨ Improved

* Added local mean sidereal time header keyword ``LMST``. Note that the value is computed without updated ``IERS-B`` tables so it may be slightly off.

✨ Improved

* Added tile pointing keywords.
* Updated `archon` to 0.11.1.

✨ Improved

* [23](https://github.com/sdss/lvmscp/pull/23) `ln2fill abort` now creates a lockfile in `/data/ln2fill.lock` that, if present, prevents new purges or fills. It can be removed with `ln2fill clear`.
* [23](https://github.com/sdss/lvmscp/pull/23) By default the pressure of the cryostats is checked before a purge or fill. If any camera is above the threshold the purge/fill is aborted.
* Custom configuration is now merged with the default, internal configuration.
* Clean-up how header keywords are compiled and how defaults are set.

🔧 Fixed

* Emit timed status messages as internal and without registering them in the log.