Cyclus Tutorial Changes

[abachma2]

The Cyclus tutorial still needs some changes, including:

• consistency in section headings, concept vs activity vs example
• consistency in key word/vocabulary formatting
• Updating source/user/tutorial/data_explorer.rst
• Move/edit discussion of DRE
• Use relative paths for pages on the website ??
• Changing/removing solution_priority from commodity discussion in source/user/tutorial/add_commod_recipe.rst

These come from comments made in PR #293 that weren't included in the PR.

[abachma2]

Another thing to add: add data analysis example using cymetric by updating source/user/tutorial/data_explorer.rst.