Fracture population separated from fracture generator

[jbrezmorf]

Fracture Population - stochastic model of fractures
Fracture Generator - particular fr size range, particular domain

Separate these two, the first one is a simply data class input for the second one.
Optionally provide direct factory functions for the second class.

Intricate interaction with position model for correlated positions.

In principle we generate fractures in a bounding box of a domain and then cut it by the domain boundary.
This cutting process is part of the geometry generation.

[jbrezmorf]

Mainly done in JB_homo branch. Setting sampling size creates a new Population instance.

TODO:

• add position part of Population as well, set unit cube as a default domain, allow setting another common domain to families similarly to size manipulations.