All external code (nlohmann, tk_spline, pybind11_json, plasimo)
now appears in subdirectories with these names in the directory
external/. That directory replaces the old directories pi_support/
and plutil/

magnumpi/consts.h now populates a namespace magnumpi::constants
with the relevant constants (pi, physical constants in SI base units),
using the contents of external/plasimo/plmath/consts.h. The directory
structure inside external/plasimo may appear to be a bit overkill,
but has the very practical advantage that it replicates the structure
of the PLASIMO code, making it much easier to do code comparisons.

The directories external and external/plasimo are in the include path,
so you can write something like '#include "nlohmann/json.h"' or
'#include "plmath/fft.h"' in the C++ code.