Sdss-lvmscp

✨ 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.

🚨 Removed

* Deleted legacy lab code to write log data and Google Sheets

✨ Improved

* The actor now emits its status on a timer with delay configurable as `status_delay`. With this `cerebro` is not required to ask for the status directly.

✨ Improved

* [21](https://github.com/sdss/lvmscp/issues/21) Improve how the delegate retries opening/closing the shutter when the first attempt fails. If the shutter fails twice closing, the exposure is read anyway.

✨ Improved

* Added ``SMJD`` header keyword with the SDSS Modified Julian Date.

🔧 Fixed

* [20](https://github.com/sdss/lvmscp/issues/20) Update archon to 0.11 which solves the issue with exposure times being limited to 1000 seconds. Marking this as a minor release because archon 0.11 is a breaking change and requires a major modification to the ACF that is not backwards compatible.