extend diag_mediator to handle many diagnostic variations/aliases

[nichannah]

Presently the diag_mediator interface can be used to save a CMOR alias of a diagnostic as well as the primary/native diagnostic. Instead of requiring separate calls to diag_register() and post_data() for CMOR the diag_mediator takes care of this implicitly. If a diagnostic has a CMOR alias it will be registered+posted at the same time as the primary diagnostic.

In preparation for #62 we'd like to generalise this approach so that a diagnostic can not only have an alias/variation for CMOR but also for vertical coordinates. So, for example a call to post_data may result in many 'variations' of a diagnostic being written out. There will be the primary diagnostic, the CMOR diag, and the another with a remapped vertical coordinate. Other variations could also be accommodated, for example a decimated variation.