Implement an Epstein function for fitting wavefronts in image data

[aringlis]

Since we may be interested in the 'steepness' in the y-direction of a detected wavefront, a good choice for a EIT wave fitting function might be an Epstein function, which is given by:

f(x) = A sech^2 [ (x-x0/a)^n]

where n is a steepness parameter. For n = 1, we have the classical Epstein profile, which has a similar shape to a Gaussian, while for n --> infinity the function tends towards a boxcar.

For extra functionality, we might implement a split function which allows for an asymmetric profile. For example:

For x < x0:
f(x) = A sech^2 [ (x-x0/a2)^n2]

For x > x0:
f(x) = A sech^2 [ (x-x0/a)^n]