Reading the NIST standard, the ridge count can be present in the
minutiae field. By simply splitting the field with the normal separator,
it's not possible, without counting the number of fields retrieved, to
know if the RC field is present or not, hence not possible to use the
variable unpacking.

This commit implements the storage of multiple possible Finger Position
Codes for the Types3-6, field 004. The field 004 is described as follow
in the NIST documentation (version 2007):

    11.2.4 Finger position (FGP)

    This mandatory fixed-length field of six binary bytes shall occupy
    the seventh through twelfth byte positions of each record type. It
    shall contain possible finger positions beginning in the leftmost
    byte of the field (byte seven of the record). The decimal code
    number corresponding to the known or most probable finger position
    shall be taken from Table12 (only finger numbers 0-14 apply to Types
    3-6) and entered as a binary number right justified and left zero
    filled within the eight-bit byte. Up to five additional finger
    positions may be referenced by entering the alternate finger
    positions in the remaining five bytes using the same format. If
    fewer than five finger position references are to be used, the
    unused bytes shall be filled with the binary equivalent of "255".
    The code "0", for "Unknown Finger", shall be used to reference every
    finger position from one through ten.

This will help the debugging process for non-valid GCA values.

This function get the list of all fpc present in the NIST file,
regardless of the idc related to the fpc. This is useful to iterate on
all fpc available.