This commit does not include any functional changes.

It partially reverts 3730f119 (ENH: Model reduction fixed up, toggleable
in cmake, default off) by switching back to using the existing "VegaFEM-CMake"
project where the master branch has been updated to include all the commit
originally associated with the "build_model_reduction" from "vegafemv4.0"
project.

To avoid inflating the size of the project, the following commits
from the "build_model_reduction" available on "vegafemv4.0" have
been squashed together while "rebasing" the changes:
* ENH: Initial commit of vegaFEMv4.0 code
* REFAC: Remove all the extra files that are not code

This allows to keep the size to ~22MiB instead of the ~126MiB.