[parallel] Add experimental support for thread parallel assembly

This adds experimental support for thread parallel assembly. As discussed this opt-in feature which is implemented by an additional template and constructor argument for Assembler for passing an Executor. If this is not specified a SequentialExecutor is used that mimics the old behavior.

For parallel execution this adds the following experimental stuff (originating in dune-fufem) to Dune::Assembler::Experimental:

• A simple advancing front graph coloring algorithm. This is purely algebraic and independent on any grid structure.
• Utilities for computing the adjacency of the element graph in a grid view, coloring the element graph, creating a colored version of the element range.
• A simplified implementation of std::barrier from C++20 since gcc-10 does not provide it.
• A ColoredRangeExecutor that provides thread-parallel execution of algorithms using a colored range based on std::thread.

Furthermore this demonstrates the usage in the poisson-pq2 and poisson-pq2-eigen examples.

Since this leads to a change in indentation in the Assembler methods, the diff seems to be quite large. The actual change is just a few lines that forward the methods body and element loop to the executor. At least locally this can be seen using git diff -w.

There is, however, one significant change: In order to guarantee thread-locality of data, the executor captures the local assembler by value and thus makes a copy. In principle we agreed that this behavior is intended. But one may argue, that this should be avoided in the sequential case. But this would make the executor usage more invasive.