Finish NcmCalc framework

[vitenti]

We should have an unified framework for computation objects. The idea is that every object that computes physical observables using models must adhere to the NcmCalc framework.

NcmCalc must be implemented as a GInterface, that way, computation objects within an hierarchy (NcRecomb for instance) can implement this interface. Currently, these objects implement a _prepare() and a _prepare_if_needed() methods, that perform all necessary work necessary to evaluate the observables. The _prepare* methods must handle all errors such that once a prepare function is called with success, it is always safe to call any _eval* method.

There is an initial implementation of NcmCalc that needs review.