Nested namespace structure in solvers

[francoishamon]

We use very long namespace names, such as ThermalCompositionalMultiphaseBaseKernels or IsothermalCompositionalMultiphaseFVMKernels.

It seems that having a more nested namespace structure might be one way to deal with it, e.g. ::geosx::flow::multiphase::kernels::isothermal or ::geosx::kernels::flow::multiphase::isothermal (we'd have to think about the order). The benefit is being able to reference any level of this hierarchy by an explicit local alias or a using directive, and thus reduce typing.

Originally posted by @klevzoff in #1225 (comment)