Fix for bump in output when enabling auto mode

[m-lundberg]

This should fix #3.

I fixed the potentially huge dt after running in manual mode for a while by updating _last_time even when in manual mode.

For allowing to set the _error_sum to the last output variable when re-enabling auto_mode so that the PID can continue from where the manual tuning "left off", I added a new function set_auto_mode() which accepts a parameter for the last output value. I also kept the old auto_mode property setter for now for backwards compatibility.

To do before merging:

• [x] Fix tests (test_auto_mode fails now)
• [x] See if any documentation needs to be updated