libs/utils/energy: Fix df indexing

Created by: valschneider

numpy.arange() was being used to index the energy-measurements df. This method has some issues where the 'stop' value may or may not be included in the returned set, i.e: arange(0.8, 1.0, 0.1) could return [0.8, 0.9, 1.0] BUT arange(0.7, 1.0, 0.1) could return [0.7, 0.8, 0.9] (not tested for these values but it is the problem being encountered)

This could thus create n indexes for n-1 values, which would cause an exception.

numpy.linspace() is more predictible than arange(), as the 'stop' value can explicitly be included or excluded (excluded by default). This commit introduces linspace().