#552 Detector non-linearity and reciprocity failure

A basic method to apply any desired non-linearity on an Image has been included to begin with. The non-linearity can be specified by a LookupTable or as a function, possibly with parameters, that can act on NumPy arrays. In addition, it also includes a feature to correct for reciprocity failure. A basic unit test to test these functionalities have also been built.
I'll address this in the FILO order:
- Docstring now says non-negative values.
- E changed to p
- That was a typo. Fixed.

For the first three points, I felt we should give a mathematical model / expression for this effect. So I don't want to write `base_flux` in the expression but have the same quantity as p/T. It is explained immediately below that it's also the base flux and while explaining `base_flux` param, it is mentioned that. In that case, using `exp_time` in the expression can become cumbersome. So `T` or `t` should work. I changed it to `t` as you recommended.