Single precision snapshot writing

[ia267]

The user can now set snapshots to be written in single precision, which will halve the snapshot file sizes. For a simulation running with single precision the snapshots will be written in single precision, but for a simulation with double precision, the user can set snapshot_single_precision = .true. to write in single precision.

SINGLE_PREC runs would not have worked with the previous code as it was always writing with double precision (adios_type_dp) - modified it now to write with single precision (adios_type_real) if it's a single precision run.

Added is_single_prec parameter to avoid writing compilation directives in various parts of the code - if this is OK might need to refactor the code to use this parameter in other parts of the code.

Closes #212