1. - Snapshot time scaled to dimensionless units when only header is read
2. - WriteColumnSnapshotFile now scales to physical units
Merge branch 'mpi' of https://github.com/gandalfcode/gandalf into mpi
Computes domain box sizes and transmits to all other MPI nodes
1. - Fixed small time scaling bug in IO
2. - Added new time derivative function in compute.py (untested)