Cleanup stencils

[CusiniM]

Describe the issue The meaning of some of the constants defined in the stencils has slightly changed over time with the introductions of fracture dedicated stencils.

Proposed cleanup Some of the constant are now redundant and should be removed while others should probably be renamed so that it's more clear how to use them.

Additional context The latest changes to the FVM assembly kernels make them only work with tpfa. If we all agree on discarding mpfa and that, if interested in adding it in the future, we will do it through the HFVM scheme, then, some additional cleanup can be done in the stencils and the MPFAStencil stencil could be removed.