Go further `Fitting::Primitive`: introduce ScalarFields and others

[nmellado]

Currently, the concept Primitive mixes methods describing a geometric entity, and a scalar field function (with a potential and a projection operator). This concept does not really represent other models, e.g., varyfolds, *jets, etc.

The goal of this issue is thus to prepare a next version that would allow to add more analysis techniques and go beyond the scalar field model implicitly encoded in Primitive.