Added Propagator base class in new file propagators.py

• Added Propagator base class in new file struphy/models/codes/propagators.py. This fixes clapp/hylife#29. It features:
  • abstractproperty: variables (replaces the list substep_vars)
  • abstractmethod: push
  • method: in_place_update
• Each Struphy propagator derives from the base class and must be added in propagators.py; the name should start with Step... as in StepMaxwell.
  • Adapted exec.py and models.Maxwell to new use of propagators
  • Renamed StruphyModels -> StruphyModel
  • Added Strang splitting in exec.py (untested)
• Updated doc:
  • Improved developers guide: shorter, adapted to StruphyModels and Propagators
  • Detailed explanation of how to add a model. This fixes clapp/hylife#28.
  • Add new theory section, shorten developers guide.
  • New section "toolkit"
  • Add source code via ".. literalinclude::" in sphinx doc, use python highlighting only for code blocks.