Enable resources to belong to several pools

Hi,
This is related to evolution proposition Exclude some resources of a pool for a given activity . As a complement, enable any resource to belong to several resource pools. Use case is to define pools as skills pools.

Example :

• Pool of developpers : R1, R2, R3, R4
• Pools of validators : R3, R4, R5, R6
• Tasks for developpers: assign the pool of developpers
• Tasks for validators : assign the pool of validators

Here R3 and R4 can be assigned both to development or validation, depending of what is best for the planning scheduling.
Without this feature, one has to define exclusive pools and optain bad scheduling solutions, or put everybody in one single pool and manage by hand inappropriate assignments, or don't use pools at all and pschedule by hand, depriving one from projeqtor scheduling benefits.

Once again, a better solution would be to introduce "skills" and "levels of skills" in projeqtor.