introduce a hmsl

[cosunae]

Description

For some datasets that we are archiving in FDB, we are combining variables defined with typeOfFirstFixedSurface 103 and 102, that is height above ground and heigh above mean sea level. For example, flight levels relevant for aviation products are defined with 102 (hmsl).
In order to be able to distinguish both, we need a different mars levtype, so our proposal would be to keep 103 with levtype 'hl'
and set a 'hmsl' for 102:

'hmsl' = {typeOfFirstFixedSurface=102; typeOfSecondFixedSurface=255;}

This would require to extend the metkit language to support it. Would that be ok?
Any other recommendation is welcome

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