Fixed demo_heat_equation notebook.

Fixed toDot.

Remove unnecessary comments.

because boundary handling does its own flag management

- sphinx detection is not reliable, e.g. jupyter notebooks have sphinx
  imported at startup -> no caching from notebooks
- explicitly added cached functions via autofunction to docs
- some functions do not support memory cache, because memory cache can't
  handle non-hashable parameters -> introduced diskcacheNoFallback
  decorator for these functions

- single function to create kernel for specified target
- data type creation from string - reuse numpy functionality
- bugfixes in dot display

Added DebugGui

- loop cutting
- simplification of conditionals inside loop

- alignment of index array when using additional data
- data types have been hashed incorrectly

Updated testing of kerncraft_coupling and some functions in
kerncraft_coupling in order to be compatible with the kerncraft version 5.1

Testcases test_mu_equivalence, test_phi_equivalence are passing.

- support for logical operators, and/or
- both phase field kernels can be vectorized now

- phasefield phi sweep vectorizes successfully

Fixed a severe bug. Renamed makePythonFunction of the llvm backend.
Deleted code duplicity.