Standardise callbacks for Nash computation algorithms

[tturocy]

This standardises the structure of callbacks for Nash equilibrium methods:

• All methods call an "on equilibrium" callback, immediately upon identifying a profile as an equilibrium (to within given tolerances as appropriate)
• Methods for which progress updates are meaningful/useful define one or more event types, which are emitted (via a callback) when they occur. These are method-specific.

Closes #340 - as this now standardises to the extent sensible calling conventions across all methods.