Additions and improvements to Bfr tutorials:

[barfowl]

These changes to the Bfr tutorials include:

• a new tutorial_1_5 to illustrate limit stencil evaluation of Surfaces
• a new tutorial_3_2 to show ways of caching Surfaces for later use
• improvements to tutorial_2_2 (more comments, simplification, some refactoring)
• updates to Bfr tutorial docs to include new tutorials and other improvements
• simplification of CMake files for all tutorials to a single call to osd_add_bfr_tutorial