Add lightning flash extent density (FED) EnVar assimilation capability

[hongli-wang]

This work supports RRFS_B GSI FED assimilation with the purpose of next operational release.
This work benefits from the David's recent PR on FED observer in GSI. But adds a new GSI EnVar FED assimilation capability.

The modification will include but not limited to:

1. Read FED background and ensemble from restart phy files
2. Add new control variable of fed ( in anavinfo, section: met guess, state and control variable)
3. Create intfed.f90 and sfpfed.f90 for minimization.
4. Other related codes. For example, update hydrometers when either dbz or fed is assimilated, or both are assimilated. Previously the update of hydrometers is done only when dbz is assimilated.

The fed assimilation should work indepently or along with radar dbz assimition. A reviewer who developed or knows well dbz EnVar assimilation is recommended to involve in the review process.