HDF5 interface and support for stochastic reading

[20DM]

First stab at an HDF5 interface + some helper functions to support stochastic reading of the file.

Different read modes are supported and documented in the HDF5 part of the tets in mpi_read_measurements.cc:

• the full data stream in the HDF5 file can be read by each rank
• the full data stream in the HDF5 file can be divided by the number of MPI ranks into even slices, with each rank only reading its designated slice
• the root rank can read the full data stream in the HDF5 file, and distribute it to the other ranks
• assuming the data stream is extremely large, such that the evenly split slices are still too large to be held in memory, a stochastic reading method is provides which will read a small chunk ("minibatch") of the slice. The starting position of the chunk is chosen randomly (uniformly distributed across the slice length, applying a wrap around if the starting position is near the end of the slice)