Allow for nested build directories

[larshinueber]

Describe the issue:

At the moment it is not possible to have nested build directories, as the build.py script assumes that the source directory is the parent directory of the build.
This is relevant if you would like to have two builds concurrently (say stable and develop) inside a single builds directory (i.e. builds/stable and builds/develop).
This is low priority, as you can simply create two build dirs at the project level (build-stable and build-develop).

How to reproduce the issue:

Run the build.py script from the project level, with a nested build directory specified:

cd tudat-bundle
python build.py --build-dir builds/stable

OS and Python version:

Should be independent, but for completeness:
Python 3.11.12
Ubuntu 22.04.5 LTS through WSL

Expected behavior:

The build should be output correctly to build/stable.

Current behavior:

~/repos/tudat-bundle main !3 ?4 ❯ python build.py --build-dir builds/stable                                                                                                                           tudat-bundle
CMake Error: The source directory "/home/lars/repos/tudat-bundle/builds" does not appear to contain CMakeLists.txt.
Specify --help for usage, or press the help button on the CMake GUI.
Traceback (most recent call last):
  File "/home/lars/repos/tudat-bundle/build.py", line 1165, in <module>
    builder.build_libraries()
  File "/home/lars/repos/tudat-bundle/build.py", line 1105, in build_libraries
    raise RuntimeError("CMake setup failed")
RuntimeError: CMake setup failed

Additional context:

No response

[alfonsoSR]

Noted :) I'll try to patch it this weekend