Implement user-defined energy steps

[martinwissink]

The ENDF database has cross section data from 1E-5 eV to 2E7 eV for many isotopes. The current implementation of linearly spaced energy steps at fixed interval is very limiting when trying to retrieve data across several orders of magnitude. Several approaches could be implemented to improve the utility:

1. Allow for the user to provide a list of desired energy points
2. Allow the user to select the number of points rather than the interval between points
3. Allow the user to select linearly or logarithmically spaced energy points