Enable firmware averaging

[crnbaker]

Many cards support on-board averaging. SPC_CARDMODE needs to be set to one of the following values:

• SPC_REC_STD_AVERAGE for averaging in standard acquisition mode resulting in 32 bit data
• SPC_REC_STD_AVERAGE_16BIT for averaging in standard acquisition mode resulting in 16 bit data (8 bit cards only)
• SPC_REC_FIFO_AVERAGE for averaging in FIFO acquisition mode resulting in 32 bit data
• SPC_REC_FIFO_AVERAGE_16BIT for averaging in FIFO acquisition mode resulting in 16 bit data (8 bit cards only)

Then you set the number of averages by writing to the SPC_AVERAGES register.