Support data with no init time in MSS server

[Joern Ungermann]

Certain data products like AIRS data do not have the split between init and valid times. For these, only the valid time should be set and the init field should be blank.

[Reimar Bauer]

Reworked structure for having init/valid times in WMS

This allows visualisation of datasets with no init or valid time. With respect to valid time, the time dimension is so far still required. For init_time, effectivly the unit string is ignored. Made the determination of elevation levels also waaaay more efficient.

Fix issue #427

→ <<cset 41fcc40a8f0b>>