Allow building with meson and meson-python

The scipy and numpy communities chose meson over CMake and migrated to meson. With https://github.com/FFY00/meson-python they have a modern and actively maintained "backend" for pip and pypa/build. Unfortunately scikit-build (the CMake-equivalent) is in much worse shape and does not seem to be actively developed anymore.

This MR adds support for meson while keeping support for CMake.

source/meson.build

+pybind11_dep = dependency('pybind11')
+eigen3_dep = dependency('eigen3', version : '>=3.4')
+
+src_solver = ['solver.h', 'solver_py.cpp']
+
+dependencies = [py3_dep, pybind11_dep, eigen3_dep]
+
+mod_solver = py3.extension_module('_solver',
+  sources : src_solver,
+  dependencies : dependencies,
+  install : true,
+  subdir : 'tmmsolve'
+  )