Adding custom input for Issue10

[hrwakeford]

This update allows the user to input a custom throughput profile.

The README.md and limb_darkeing.py files have been updated to include new parameters for this.

The function also works as previously when known modes are called without the need of the new keywords

[DavoGrant]

This looks great. One question: I am wondering if the wsdata input should only have two values (start, stop) corresponding to the wavelength range? np.array([start, stop]) rather than np.arange(start, stop, step)?

[hrwakeford]

The wsdata normally corresponds to the computed wavelength solution of the spectra over the wavelength range you want to consider for your light curve. So the example used here is just a simple array with a step size, but in reality it would likely be a truer representation of the wavelengths considered for that specific light curve.

For example in ExoTiC-ISM you can see that the input wsdata is an array of wavelengths computed for the data - in that case the broadband white light curve.

So, while this can be an arbitrary array such as in the example, it can also be more specific for that dataset.