gnuradio-core: export missing public methods in gr.agc2

gnuradio-core: export missing public methods in gr.agc2

Alexandru Csete:

The gr_agc2 block has public methods to set and get the AGC levels and
rates; however, only a few of these interfaces are exported to Python
and none of them are available in GRC.

The attached patch adds the missing public methods to gri_agc2_cc.i
and gri_agc2_ff.i and defines the corresponding callbacks in the
gr_agc2_xx.xml GRC block.