Add LDTM to allow for filenames where lead time is in minutes

[j-fannon]

Presumably something better should be done, or has already been done?

[andrew-MET]

Yes - the idea is that user will specify the units in the lead_time argument. If numeric, it'll be assumed to be in hours so as not to break the current behaviour, otherwise a unit could be added using "s", "m", "h", "d", probably using one of the seq_secs(), seq_mins(), seq_hours() or seq_days() functions that already exist. The lead time units in the filename will be assumed to be the same as those passed to lead_time.

This ultimately means that lead times will be stored in seconds rather than hours as they currently are, so there's quite a bit of code that will need changing to accommodate this.