Modernize and generalize the code of some Spectrum methods, rename some of them, some style corrections, a bug fixed and a Signal method removed.

* Rename Signal.swap_axis to swap_axes
* Remove Signal.iterate_axis and utils.iterate_axis
* Rename Spectrum.align_with_array_1D to Spectrum.shift_1D
* Merge Spectrum.interpolate_in_index_1D and interpolate_in_units_1Dt in the new method interpolate_in_between that makes use of the new float/int syntax for indexing axes.
* Merge Spectrum.estimate_shift_in_index_1D and Spectrum.estimate_shift_in_units_1D in the new method Spectrum.estimate_shift_1D that makes use of the new float/int syntax for indexing axes.
* Add extra aligning test
* Make the methods that require a signal_dimension == 1 to raise an exception when this is not the case.
Move methods from Spectrum to Signal and instruct git to ignore spyder config file.