Complement documentation.
Add time dependence to eval(), eval_gradients() and mc() methods.

The eval_gradients() method is no longer const to reflect the fact that the method explicitly sets the gradient attribute of the model parameters.
Adapt to new spectral class interface.
Update documentation
Adapt to new GModelSky interface.

The value() method now takes a photon as argument.
Complete value constructor, add member access methods and introduce precomputation cache.

The value constructor now takes values for all 4 parameter members as arguments. The pivot and cut-off energy arguments have been declared as GEnergy.

All parameter members now have set and retrieve methods. The pivot and cut-off method set and retrieve GEnergy objects.

A precomputation cache has been added for the eval() and eval_gradients() methods that does accelerate computations in case that the energy and the index, cutoff and pivot energy parameters have not changed. This should improve the performance for binned analysis.
Flux variable in eflux() method now defined as eflux.
Correct documentation.
Modify value constructor, add methods for parameter access, add evaluation and mc cache.

The value constructor now takes an object of type GEnergy for the pivot energy.

Methods have been added to access are three model parameters.

Evaluation caches have been added to support the eval(), eval_gradients() and mc() methods. This speeds up evaluation of the spectrum, in particular for binned analysis, and the simulation of photons.
Correct documentation.
Adapt to new GModelSpectral interface.