Support loading of LBC type fieldsfiles.

Modifies existing ff module to treat LBC files specially :
 * various key values are assumed as part of the format + not recorded in the lookup headers
    * this means especially LBTIM, LBVC, which concepts have a specific interpretation for LBCs
 * there are multiple 2d fields per lookup table entry (alternatively, they have 3d data)
   * we are returning this as a sequence of 2d fields, to be compatible with the existing loading code
   * the usual per-level layer coordinate data is absent - for now, only LBLEV varies and the rest (blev/brlev/brsvd1 and bhlev/bhrlev/brsvd2) are all 0


> Where is the requirement for LBTIM == 0 documented? Similarly, where is the documentation for the expectation that it should be treated as if LBTIM == 11? (I can find no mention of this in F3 or C71.)

Quite right, there is none.
I have an email from Stuart Whitehouse stating frankly that the documentation for the LBC format is desparately lacking, if that helps at all...

For now, I have simply "made up" all these assumptions about how the metadata encoding works, based in what I found in the test files.
Those assumptions are presented here as appropriate "equivalent" values for LBTIM and LBVC.  We need to feed this back to those in the know, hopefully the UM Utils people.