RENAME: components.voigt.Voigt.origin -> Voigt.centre
BUG-FIX: in EELSSpectrum the global elements variable was being overwritten in the add_elements method. Bug reported by Magnus Nord in issue #58
Add inverse_selection flag to model.fit_component

Fits the component to the signal which is not selected
by the user input, for example when fitting an arctan.
BUG-FIX: get_fine_structure_as_spectrum in eels_cl_edge component
Merge branch 'master' into gui_fit_model_components
Change x0 parameter in arctan to centre
Add y0 parameter to arctan component

Need this to enable use of arctan component in the fit_component
function in model.py
Add setting of more initial parameters in ComponentFit

ComponentFit will set initial values for the fitting for
sigma and A parameters. Implemented for Gaussians, have not
tried on other components.