This is useful when preparing packages for Linux distributions.

CI: optimized tags for build jobs

See merge request !103

The build directory for all build types is grouped under a top-level
build directory called "builddir", i.e. "builddir/Release" etc.

Added Code of Conduct

See merge request !102

Fixes #86

This fixes the previous commit.

These functions expect all index parameters to have the same type, so we
need to cast 0 to the IndexType, otherwise the function lookup will fail
when IndexType is not int.

Added __device__ flag to the Shared union constructors in CudaReductionKernel.h and CudaScanKernel.h to make clang happy

Addresses #86

Also fixed the header in Preconditioner.h

MPI refactoring

See merge request !101

It does not make sense to print a distributed array like this, because
it may contain ghost elements. Users should examine the local array
view manually.

Replaced send/receive for Array(,View) and mpiSend/mpiReceive for String with a general implementation in the MPI namespace

Also added analogous functions: MPI::sendrecv, MPI::bcast.