Gradual amplification coefficient change

[JanKuczma]

Enforce gradual change of A. There are two additional constants that control how the A can be changed:

• MIN_AMP_DURATION - The minimum period (in milisec) for ramping A. Set to 24h.
• MAX_AMP_CHANGE - How many times the A can increase or decrease over one ramping period (at least MIN_AMP_DURATION).
  E.g. the current A is 100, and the MAX_A_CHANGE is set to 10. It means the A can be changed to a value in range 10 to 1000 (current_A / MAX_A_CHANGE to current_A * MAX_A_CHANGE).
• A is also restricted with MIN_A and MAX_A.

[woocash2]

Looks good.

I only had one concern: whether someone could enforce too fast amp_coef change by repeatedly calling ramp and stop_ramp. But it seems that because amp_delta is rounded down due to division, it can't happen.