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


I thought it was reasonable as-is, given the exisitng structure: _extract_fields is rather long and rambling, so inserting extra processing steps would mean refactoring to split it up, which would be needed to produce a LBC-specific subclass with modified behaviour.

Given all that, I thought this was a reasonably well contained solution that doesn't add too much complexity to the 'normal' code path.

*However* I've just uncovered a problem with the data payloads (see below) which at first sight will add more complexity impacting this, so that might change my decision.

See on below....