Removed hard-coded build-time venv.

This unifies the doxygen headers. If the (doxygen) header is not
starting with \file, the following documentation is attributed to a
different entity resulting is multiple copyright blocks and the like in
the doxygen's html files.

Allows ogs to be interrupted by SIGINT, which otherwise is handled by
python. See
https://docs.python.org/3/c-api/exceptions.html#c.PyErr_CheckSignals and
https://pybind11.readthedocs.io/en/stable/faq.html#how-can-i-properly-handle-ctrl-c-in-long-running-functions

Include spdlog via BaseLib/Logging.h, which
provides macro replacements for
ERR/DBUG/WARN/INFO...

Add BHEinflowPythonBoundaryCondition into following places
1) PythonBoundaryCondition
2) Pybind11_Embedded_Module
3) The CMakeList
4) Switch on the compiling flag OGS_USE_PYTHON when pipe network is present
Add OGS_USE_PYTHON option in HeatTransportBHE process cmakelist

The warning is issued by MSVC:
Compiler Warning (level 3) C4197.
type' : top-level volatile in cast is ignored