IF summary

[astrofle]

Feature description
I would like to know how the IF numbers map to frequency.
For example something like

sdf = GBTFITSLoad()
sdf.if_summary()

shows how the IF numbers map to frequency. One possibility would be to get something like this back

{"1420 MHz": 0, "1665 MHz": 1, "1720 MHz": 2}

so then we can do

sdf.getps(ifnum=sdf.if_summary()["1665 MHz"])