Change British spellings -> American spellings for consistency

[jchodera]

I found a few British spellings in the API:

• minimisation_steps should be changed to minimization_steps
• minimise should be minimize

[ijpulidos]

Directly changing these will break the API compatibility. Considering that this code is still experimental and in a 0.x version that is subject to API breaking changes, I think this could be okay for our 0.11 milestone release. This does not follow semantic versioning but it is not that disruptive, since we probably expect many API changes in that release anyways. Does that sound okay?