Set max and min amount of person with a given role in entities specification

[Anna-Livia]

Rewritten by @fpagnoux

As a country package maintainer, I'd like to be able to specify, for each role of an entity:

• How many people can have this role?
• If this role is mandatory

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Currently, I can specify a max but it doesn't work well: if I send to many people with this role, I get a very non-explicit error:

    yield role.subroles[legacy_role_j], legacy_role_i + legacy_role_j, individu
IndexError: list index out of range

There is no way right now to specify that a role is mandatory.

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Suggestion:

• Properly enforce the max of a role, and send a nice error message when a user tries to send more
• Introduce a min properties of role, and handle it the same way

(I have a slight preference for min over mandatory as it's more consistent with max and a little more versatile, but up to debate)

[Morendil]

Closing this as #782 is the same issue but the discussion has much more detail.