[REF] resource: correctly take into account timezones and clean code

Currently timezones were not or incorrectly taken into account in resource
computation. Indeed complexity comes from attendances being expressed in
naive timezones valid for all users, whereas dates and leaves are computed
into UTC. Computation should always be done in user timezone so that there
are no overlapping days and all time intervals are correctly computed.

A new timezone field is added on leaves to know in which timezone they
were originally encoded. Indeed putting them in naive user timezone
require its original timezone, not the current user timezone, as those
can be different.

This commit also contains code cleaning in resource to be a bit more
pythonic, simplify method parameters and offer a more simple api to
other modules.

Eventually code field is removed on resource as it is not really necessary
for resources. It is explicitly added on workcenter model who use it.

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