`extern crate lib_blas` in test_parallel_grid

[mscroggs]

Currently, including lib_blas it as an external crate in examples/test_parallel_grid.py gives an error, see eg https://github.com/bempp/bempp-rs/actions/runs/9020467149/job/24785684472

It appears that this is only an issue when using cargo mpirun and only fails sometimes: https://github.com/bempp/bempp-rs/actions/runs/9020456872/job/24785654023 passed and https://github.com/bempp/bempp-rs/actions/runs/9020467149/job/24785684472 failed, and the only difference between those two runs was cargo fmt

Steps to reproduce error in empty ubuntu docker image:

apt update
apt install pkg-config build-essential curl git nano cmake gfortran python3-dev clang libmpich-dev \\
    liblapack-dev libblas libblis-dev
curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
git clone https://github.com/bempp/bempp-rs.git
git checkout fad3934a5b11cf247e803e93cea00db558dada6a
cd bempp-rs
$HOME/.cargo/bin/cargo install cargo-mpirun
$HOME/.cargo/bin/cargo mpirun --example test_parallel_grid --release -n 2 --features "mpi"