Add CMake Presets

[med-ayssar]

NEST has no CMakePresets.json, forcing every developer to manually remember and type the same long -D flag chains every time they configure a build. This adds friction for new contributors and makes it easy to misconfigure.

This PR adds CMakePresets.json with 7 presets covering the most common build scenarios (default, debug, release, mpi, mpi-debug, minimal, docs), so developers can replace verbose cmake -Dwith-mpi=ON -Dwith-optimize=ON ... invocations with a single cmake --preset <name>.

CMake Presets

Preset	Description	Key flags
default	Standard dev build	-O2, OpenMP, Python, GSL, Boost
debug	Debug build	-g, no optimization
release	Optimized release	-O3
mpi	MPI + OpenMP	-O2, MPI enabled
mpi-debug	MPI debug	-g, MPI enabled
minimal	No optional deps	No Python/MPI/Boost/GSL
docs	With user docs	Requires Sphinx + Pandoc

Each preset puts its binary dir under build/<preset-name>/ to avoid conflicts.

List all presets

cmake --list-presets

Configure

cmake --preset default
cmake --preset debug
cmake --preset release
cmake --preset mpi
cmake --preset mpi-debug
cmake --preset minimal
cmake --preset docs

Build

cmake --build --preset default
cmake --build --preset debug

Install

cmake --build --preset default --target install

Note: you need to run the configure stage first, so that it creates the buildDir!

Example

cmake --preset debug
cd build/debug
make install -j7

[med-ayssar]

@heplesser, @terhorstd and @gtrensch: could you maybe give it a try and approve it if you see it fit.