Un-setting profile pins and commands

[carterturn]

If dds sweeper is not in all channels mode, profile pins are set with |= in set_ins_sweeps. Thus, if a profile pin bit is set high for a command (either due to a past command, or due to switching modes and changing the interpretation of some bytes in instructions), it is only possible to set it low by creating a stop command at that address, then re-adding the other commands.

It would probably be useful to have a way to reset the profile pin for a single channel in a single command.

Relatedly, it would be useful to have "null commands" for certain channels, so that the other channels could be updated. An easy way I see to do this might be to use read instructions instead of write instructions (and discard the results). This would probably require some changes to the set/seti/setb syntax.