Cleanup submodules structure

Submodules structure

Refactor our modules structure similarly to SciPy modules.

Main submodules are:

• hotopy.holo (rename from hotopy.phase to be consistent to holo-tomo) including propagation (currently in hotopy.propagation) and reconstruction methods Existing helper routines should be placed in private submodules not meant exposed via the public API (e.g. hotopy.fourier).
• hotopy.tomo for tomographic projection and reconstruction (including ASTRA, tomoPy, CIL wrappers)
• hotopy.optimize replacing the algorithms module. Mainly for internal use but should also allow general purpose use.
• hotopy.image for image manipulation helpers.
• hotopy.datasets remains as is.