Merge branch 'fix-hdf5-errors' into 'master'

Use H5Lexists to check whether /Units exists in IC files before attempting to read them

Updated to use H5Lexists to check whether user has input units into 
their Initial Conditions files. This avoids error rasises from H5G* when
opening files without units and reverting back to the ones specified in
the parameterfile. We now also let users know when their units are being
read from their ICs explicitly.

See merge request !427