Unify redundant ECE metrics

[perellonieto]

There are multiple functions to compute ECE because of legacy reasons.

• ECE: https://github.com/perellonieto/PyCalib/blob/master/pycalib/metrics.py#L137
• conf_ECE: https://github.com/perellonieto/PyCalib/blob/master/pycalib/metrics.py#L111
• binary_ECE: https://github.com/perellonieto/PyCalib/blob/master/pycalib/metrics.py#L436
• classwise_ECE: https://github.com/perellonieto/PyCalib/blob/master/pycalib/metrics.py#L450
• full_ECE: https://github.com/perellonieto/PyCalib/blob/master/pycalib/metrics.py#L500

The first one had different options to compute confidence-ECE and other variants. I think we can remove the general function ECE and only use the more specific ones, but this needs to be clarified and do properly.