Add error message if user_nl_blom includes hash comments

[TomasTorsvik]

If someone use an old user_nl_blom file with hash symbols (#) for comments, there will be an error message on the form:

ocn_in_paramgen.OcnInParamGenError: Line number 1 in 'user_nl_blom' starts with a hash sign (#). This may indicate an outdated format of your
user_nl_blom file. Current versions of user_nl_blom file conform to the standard Fortran namelist syntax, indicating comments
with exclamation mark (!). Please check that your user_nl_blom file conforms with the current accepted file format.

This does not guarantee that older files will be caught, only in case comment lines are kept.
Response to NorESM issue #618.

If this PR receives positive reviews, I would include the same error message in release-1.6 and master branches as well.

[JorgSchwinger]

Looks good to me (I'm not very proficient in Python though)

[jmaerz]

Looks good to me as well - thanks for providing this error message!

[matsbn]

Looks good, thanks!