ExtraFitsKeywords should allow defaults

[hugobuddel]

It would be nice if the ExtraFitsKeywords effect could have default values for the headers if the ! and # keywords do not resolve.

#61 adds several header keywords for MICADO. Many of them come directly from the OBS dictionary. It would be nice if these would have defaults, then it is only necessary to specify them if they deviate from the defaults.

TBD what the best way to do this is. Note that we have to provide Data Interface Dictionaries for these header keywords, which can contain default values (as well as units etc), so maybe that would be the easiest because we do have those: https://gitlab.astro-wise.org/micado/micadowise/-/tree/develop/src/datainterface

[hugobuddel]

Duh, the defaults can be defined in, tadadadada, default.yaml.