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.
Add new evaporation spectrum sampling method outlined in LA-UR-14-27694. Still
need documentation.
Update documentation for evaporation spectrum.
Merge pull request #337 from paulromano/evaporation

Improved evaporation spectrum sampling algorithm
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.
Mention ERSN-OpenMC in user's guide section on writing input files.
Don't allow source energies above 20 MeV for the time being.
Prevent monoenergetic source at exactly 20 MeV