Fix flog deconvolution

Dear Everyone, 

I have created this new branch as a last follow-up on the ZLP_removal_tool branch. The name is badly chosen, as this branch does not substitute the fourier-log deconvolution routine, although it changes slightly its behavior, complementing it with new capabilities and leaving untouched the old ones. 

This branch incorporates a new method for splicing the ZLP that is called splice_zero_loss_peak_flog. It is not only used for splicing ZLP from the rest of the spectrum, but also to prepare these spectra for fft treatment in f.log deconvolution, following techniques explained in Egerton (2011). 

<<Both EELSSpectra (input and output ZLP) are expanded to the next power of two, and the left side of the ZLP  is moved to the right end. The discontinuities are smoothed using Hann window, preserving the value of the integral below the spectra. Input EELSSpectrum is extrapolated to the right using power_law_extrapolation.>>

Then the fourier_log_deconvolution has also been slightly modified to be compatible with the treated EELSSpectra. The old behavior is conserved.

I am currently testing the routines, but I'll leave here the branch for if someone wants to join :)

Best wishes,
Alberto.