Membership: Make membership_type argument optionnal and by default value 'IN'

In the Membership class, the membership_type argument/attribute can be of IN or OUT value. The IN value is the standard way of sending a membership. The OUT is almost never used, and was meant to cancel a membership, without sending a revocation for an identity.

• Protocol defining the Memership document

Findings on the forum:

• https://forum.duniter.org/t/protocol-implicit-revocation/689/5
• https://forum.duniter.org/t/simplification-de-la-machine-a-etat-des-identites/4872/4
• https://forum.duniter.org/t/tortue-faisait-du-multi-comptes/1386/10

Let's define it as optional, since we might get rid of it, in case the revocation took over this use case. It would then be easier to just remove the argument from the prompts of Membership methods. Not sure if we would get rid of it since, there is a leavers field in the block document containing the OUT membership documents.

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Todo

• Make it optional and by default value IN.