Update package layout

[MaxvdKolk]

Update the package layout to fit with the typical package layout. There is already a basic skeleton, but I can attempt to move the code more into that layout.

docs/
  build/  # default output location of the (automatically) constructed docs
  source/  # the source for the documentation 
sao/  # all the code related to the SAO project
  problems/  # all the test problems we would like to provide with the package
  approximations/  # the different approximation schemes
  solvers/  # the subsolver routines
tests/  # test scripts to assert performance of the code

[Giannis1993]

Done!