Update project structure

[ots22]

• Renames the 'intake_cryoem' module to 'empiarreader' to be consistent with the package name
  • the original motivation for this was that intake driver packages/modules are often called intake-something/intake_something, but we already break from this with the package name anyway (which I think we want to keep) and consistency seems more important
• MrcSource, StarSource, EmpiarSource and EmpiarCatalog are now available from the top-level module (that is, empiarreader.StarSource rather than intake_cryoem.starsource.StarSource)
• Keep the src directory introduced by #20

Any code with import intake_cryoem will need to be updated (to import empiarreader), but the intake interface (e.g. intake.open_mrc()) won't require any changes

[JatGreer]

This all looks good to me! @mooniean would like to make sure you're happy with our recent reorganisation of the codebase rather than merging this myself

[ots22]

Yes very happy!

@mooniean what do you think about changing the name of the module?