Remove use of eval

[DanPorter]

Request by @PeterC-DLS to remove evil use of eval in HdfMap.

eval is potentially dangerous as a user input as it can allow many un-safe usages. Currently the code contains several checks to stop this, but it is not a great way of doing this. A better (though they don't provide guarantees) way of doing this is to use the asteval module.

The hdf_eval function should be updated to use asteval rather than eval.