Add new energy grid search based on an equal log-spaced mapping technique as
outlined in LA-UR-14-24530.
Change logarithmic energy grid to default.
Update documentation for <energy_grid>.
Merge branch 'develop' into log-energy-grid
Merge branch 'develop' into log-energy-grid
Merge branch 'develop' into log-energy-grid

Conflicts:
	src/input_xml.F90
Remove union energy grid treatment.
Merge branch 'develop' into log-energy-grid
Update documentation to reflect no use of union energy grid.
Fix headings in cross section documentation.
Merge branch 'develop' into log-energy-grid
Make number of bins for log grid adjustable by user.
Add documentation for log_grid_bins.
Move energy grid related variables to energy_grid module.
Don't allow source energies above 20 MeV for the time being.
Prevent monoenergetic source at exactly 20 MeV
Merge pull request #335 from paulromano/log-energy-grid

Add equal-lethargy spaced energy grid mapping method