Allow to use immutable message list objects

[sandorkertesz]

Is your feature request related to a problem? Please describe.

Currently, unpacking a message internally goes like this:

msg["unpack"] = 1

and message object is required to be a mutable mapping The idea is that pdbufr should also be able to use immutable mappings having the equivalent unpack method:

msg.unpack()

Describe the solution you'd like

No response

Describe alternatives you've considered

No response

Additional context

No response

Organisation

ECMWF