Allow for different observation mode than POINTING in DL3 HDU

[morcuended]

cta-lstchain/lstchain/high_level/hdu_table.py

Line 49 in 620d0a9

OBS_MODE = "POINTING"

I guess we also have to update our DL2 to DL3 tool to allow setting obs_mode to drift upon request.

Originally posted by @moralejo in #1293 (comment)

We should allow the user to set the observation mode in the header of the DL3 files to something different than the current default "POINTING" (e.g. "DRIFT").