Header / Observation validation

Improve header / observation validation code to

• check all keywords if they are present or empty
• report such missing values in the header.validationLog field
• report any missing instrument mode (db query) as it should be fixed in the obsportal configuration or the wrapper code (mapping rule)
• flag the Observation (valid = True) only if it is complete (to be defined). Up to now mandatory / optional keyword lists are hard-coded but it should be more precise and updated according to the instrument.

Test:

• 1 (empty) header per instrument having only DP_CAT, DP_TYPE ... to be considered as an observation