compute_cve replace the interpolation step

[yucongalicechen]

We have an issue in diffpy.utils (diffpy/diffpy.utils#230) that interpolate onto a master grid. If that ever gets implemented, we want to replace the related steps in the compute_cve function in functions.py (see discussions in #92 and #87 for more detail).

[sbillinge]

@yucongalicechen please review and close if necessary

[yucongalicechen]

@yucongalicechen please review and close if necessary

I don't think this can be closed now. I updated the description to make it clearer.