Oem

Key Changes

* Fix astropy epoch truncation

Key Changes

* Fix unhandled exception when CCSDS_OEM_VERS is not on the first line.

Key Changes

* Significantly optimized parsing and load times. Large KVN ephemeris files should load up to 95% faster with greatly reduced memory usage.
* Added initial support for creation of OEMs from TLEs. Requires `sgp4` package and can be installed with pip install `oem[tle]`.
* Added initial support for OEM comparisons by subtracting two OEM instances with matching frame and center parameters.

Key Changes

* Added fix for handling of namespaced XML OEM files.

Key Changes

* Implement missing support for YYYY-DDD epoch format
* Prevent numpy warning messages

Key Changes

* Improved efficiency of KVN parser for large OEM files