Improved wavelength solution

This version corrects a wavelength shift of ~0.1Å across the slitlets in the final reduced cubes, so that the final cubes are now as precise as advertised during the wavelength solution derivation step.

Bad Bug fix

A debugging command was left over by mistake in the code. It has now been removed.

Misc. updates

This release includes minor fix and updates to the pipeline. The Cu-Ar (B3000) reference arc file was replaced with a (much!) more accurate version donated by I. Seitenzahl. mpfit was updated to remove some annoying warnings. Same for "header.update" replaced by "header.set".

The installation of the pipeline is now also easier, as the files auto-detect their location. All is required if to add the pywifes folder to the Python path. No need to define any global variable (pywifes_dir) anymore !