fec: wip: there's some circular importing between fec and digital.

Trying to avoid a conflict by seeing if it's already installed and just grabbing a copy called 'digital'.
digital: modified tagged stream correlate access code.

Decodes the header of a packet to get the frame length (in bytes). Uses this, fec rate, and fec extra parameter to set the tag stream output size in bits (or soft decision floats)