Restructure sub-modules

See #37 (closed) for details.

Sub-modules

The following sub-modules remain:

• hotopy.holo (former hotopy.phase merged with hotopy.propagation)
• hotopy.tomo (merge of hotopy.tomo and hotopy.astra)
• hotopy.image
• hotopy.datasets
• hotopy.utils (merge of hotopy.fourier, hotopy.xray, hotopy.utils, hotopy.io)

Sub-modules support top-level imports

(Most) Sub-modules allow convenient imports from their respective top-level, e.g. from hotopy.image import GaussianBlur, from hotopy.holo import simulate_hologram etc.

Closes #37 (closed).