laddu-python-v0.3.0

Added

• update MPI code to use root-node-agnostic methods
• first pass implementation of MPI interface

Fixed

• forgot to update the laddu-python use_mpi function to have a trigger arg
• add feature flag to laddu-python and update MSRV for mpisys compatibility

Other

• update all documentation to include MPI modules
• add some clippy lints and clean up some unused imports and redundant code
• (vectors) use custom type for 3/4-vectors rather than trait impl for nalgebra Vectors