feat: decouple vsl.mpi from vsl.la (#194)

[ulises-jeremias]

vsl.mpi is a mandatory dependency of vsl.la even though MPI is optional and
requires system-wide installation. This forces all LA users to install MPI.

Task

1. Make the MPI import in LA lazy/optional
2. Add mpi/basic_test.v that skips when mpirun is not available
3. Verify LA still works without MPI installed

Reference

• Issue: Remove vsl.mpi from vsl.la dependencies (making it optional) #194