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

**NB.** The current state of play with LBC files on "master" is: only one field is generated for each 3D lookup entry, and access to the `data` of that field is broken. This PR only addresses the first of those problems, so now multiple fields are generated for each 3D lookup entry, but access to `data` is still broken.

I was just touch-and-go on that, but as you've raised it I'll revert !