Common optimizer interface

[uschwes]

We started having different optimizers in the framework, a common interface would be great.

Suggestion for class hierarchy:
OptimizerBaseInterface - > OptimizerSquaredInterface - > OptimizerScalarInterface

Optimizers on scalar error terms can also operate on squared error terms.

Suggestion for interfaces:

• a common configuration interface can be achieved via sm::property_tree. @HannesSommer or we take the new value_store
• a common termination criterion could be achieved via a callback mechanism with a default implementation checking gradient norm, maximum change in design variables and potentially runtime.
• common status/return value interface should give access to convergence status, number of gradient and objective evaluations, current gradient norm and objective function value.

[pfmark]

Yes, I agree. I think especially having the possibility to work with both scalar and squared error terms at once is important.

[uschwes]

Addressed in #150