Add get_error_sum and reset_error_sum methods to PID class

[robberwick]

This PR introduces two new methods to the PID class: get_error_sum and reset_error_sum.

The get_error_sum method allows users to monitor the error sum or reset it as needed during runtime. For example, the getter would enable users of the library to know when a wheel has locked up or power has been lost, and the reset method would provide the ability to remediate that error state.

resolves #934