Add a parameter to choose the number of agent (Individual) when using built-in synthetic population generation

[chapuisk]

In the current version of the COMOKIT there is no way to specify the number of agent. It is always determined by the number of buildings (one household per flat in the building, 1 by default) and random rules on household composition. It could be great to have a parameter to limit the number of agent and/or household in the model !