Production release of p4est

This release is largely backwards compatible except for some fringe functions.
The configuration with MPI but without MPI I/O has been deprecated.

Production release with API version 2:0:0

Compatibility release with API version 1:0:0

Updated historic compatibility release

Streamlined build system, CI, CMake build

Improved portability, warning fixes

Tag after some build/config improvements

v2.3.1 with small improvements to legacy branch

The v2.3 series is backward compatible to the old days.
In particular, we maintain the fortran, blas and lapack code.
Those and several other dependencies to be removed in v2.8+.

Update p4est with backward-compatible dependencies

The code used for the measurement of the quaddrant loop in p4est_new_ext