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 do think it is wise to at least _check_ these settings, as the code is assuming so much that is nowhere definitely explained.
For example, if someone decides you *can* have LBC files with a different style of time encoding (LBTIM), then we should catch that rather than just ignoring the value and continuing with our own assumed interpretation.
If you don't like a warning, then to my mind throwing an exception would be better than quietly ignoring an unexpected control value - How would you feel about that @rhattersley ?