AGC tuning change and enable AGC for SBA SPAR 1TC

This commit has two changes:
A tuning change has been done to fix a clicking artifact (of which is described in the issue #168.)
The changes include:
- Limiting the attack/release done per frame to 6dB
  - In the code, this attack/release amount is configurable at compile time using the `DBSTEP` macro
- Always distributing the total amount of attack/release to be done across a maximum of 3 frames
- Removing hard transition cases and instead always doing a smooth transition

These changes are bit exact for signals that do not become OOR inside the encoder. However the changes are, of course, not bit exact for non-OOR signals.

AGC will be enabled by default when 1 transport channel is used and in SBA mode. This translates to bitrates of 24.4 and 32kbps in SBA. It is enabled only in these modes, because these are the modes where we know out of range signals can occur, and because degradations are known to occur for the 1 transport channel case. Further expla...