Mpi Debug

[AstroPatty]

• Collections now only raise ZeroLengthError if all children are length 0
• Writing now supports different structures on different ranks
• Extensive typing fixes
• Fix frozen models
• Update header region handling based on changes
• Add an additional test
• Bump Astropy version
• Add an MPI timeout to tests
• Add an MPI timeout to tests
• Fix for simple index projection
• Fix a bug for visiting structure collections
• Switch to OpenMPI
• debug