Add TLE components to the model
Add TLE panel to the GUI
Add XML element start check for TLE system

The TLE system doesn't have any attributes so the "normal" XML
processing methods don't seem to be being called.
Add TLE widgets to the GUI
Read XML elements based on current system

The orbital elements and TLE systems share some XML elements
so they must be distinguished by knowing what the current system
is.
Add TelescopeUtil feature configuration for TLE

This configuration indicates that FEATURE_TLE_SYSTEM is
supported by UKIRT but not by JCMT.
Disable TLE selection for telescopes without it

This uses the new FEATURE_TLE_SYSTEM to determine whether the TLE
system should be selectable or not.
Use names for TLE M and n components

The names are abbreviated to fit in the coordinates panel.
Merge branch 'tle'