Add ln(E/Epivot) value to precomputatiion cache.

This value is needed in the eval_gradients() method and takes up most of the computing time. Including it in the common eval() and eval_gradients() pre computation cache makes eval() slower in case that the method is called subsequently with different energies or parameters.
Instrument name and observation identifiers are now case sensitive.

There is no reason for having case insensitive instrument names and observation identifiers, a part from preventing mis typings. But type conversion is quite time consuming, thus I decided to remove all implicit type conversions from the code to speed up the computations. This is particularily relevant for the isvalid() method that is called pretty often.
Store reference to model parameter to avoid repeated operator access class in model_grad()
Use pi/2-asin() instead of acos() in arccos() function as on Linux systems this is faster.
Add arccos() method documentation
Declaire ininfinite() and isnotanumber() inline.