Finalize global library structure

[julienfausty]

The library needs a general structure to organize it's different functionalities. Here are my thoughts on the question as of now:

• core: base module that handles low level stuff (definition of arrays, specific data structures, generic algorithms, etc.)
• spaces: a module for defining abstract mathematical sets and spaces in order to stay true to the theoretical basis for finite elements
• discretizations: a module that handles discretizing spaces and equations and providing data structures to hold the discrete objects
• solvers: a module that deals with solving linear or non-linear systems
• workflows: a module that proposes pre-configured pipelines for setting up a calculation using the other modules

This is just a proposal and warrants a pretty thorough discussion. Don't hesitate to comment!

@achilleasev @mkochat