BUG : fixed miss-use of scipy.integrate.simps
BUG : fixed miss-use of scipy.integrate.simps

The input values to scipy.integrate.simps are of
the form y = f(x) and returns integral_xmin^xmax f(x),
thus passing in the left bin edges will result in computing
the integral with the range shifted by half the distance
between the bins.

To address this did two things

 1. changed documentation of integrate_ROI to reflect
    what is really happening.  This also eliminates the ambiguity of
    what to do about selecting the range, you select all points in the
    range.

 2. added `integarte_ROI_spectrum` which converts `bin_edges` (N
    left edges + 1 right edge -> bin centers and then calls
    `integrate_ROI`

scikit-xray/scikit-xray,licode/scikit-xray,scikit-xray/scikit-xray,danielballan/scikit-xray,danielballan/scikit-xray,giltis/scikit-xray,yugangzhang/scikit-beam,giltis/scikit-xray,tacaswell/scikit-beam,CJ-Wright/scikit-beam,celiafish/scikit-xray,celiafish/scikit-xray,licode/scikit-xray,Nikea/scikit-xray,tacaswell/scikit-xray,yugangzhang/scikit-beam,yugangzhang/scikit-beam,licode/scikit-beam,tacaswell/scikit-xray,tacaswell/scikit-beam,hainm/scikit-xray,giltis/scikit-xray,scikit-xray/scikit-xray,CJ-Wright/scikit-beam,Nikea/scikit-xray,Nikea/scikit-xray,ericdill/scikit-xray,licode/scikit-beam,hainm/scikit-xray,danielballan/scikit-xray,celiafish/scikit-xray,licode/scikit-xray,tacaswell/scikit-xray,tacaswell/scikit-beam,hainm/scikit-xray,ericdill/scikit-xray,ericdill/scikit-xray,CJ-Wright/scikit-beam,licode/scikit-beam