Added initial version of 2D gravity example
Added README.
Delete log files I didn't mean to include.
Removed spatial database files that are created by python script.
Finished setting up gravity example.
Need to have Brad and Matt test it.
Small improvements to 2-D gravity example.

Fixed location of inner block. Adjusted solver tolerances (KSP smaller
than SNES).
Merge branch 'baagaard/fix-meshing-examples-trelis' into next

* baagaard/fix-meshing-examples-trelis:
  Updated geometry journal file based on updated surfaces (change in ids).
  Updated meshing README.
  Fixed typo.
  Improve subduction meshing example. Make Trelis compatible.
  Removed obsolete contour meshing example (covered by 3-D subduction).
Merge branch 'williams/add-examples-grav2d' into next

* williams/add-examples-grav2d:
  Small improvements to 2-D gravity example.
  Finished setting up gravity example. Need to have Brad and Matt test it.
  Removed spatial database files that are created by python script.
  Delete log files I didn't mean to include.
  Added README.
  Added initial version of 2D gravity example
Started work on debugging examples.
Added files for examples/debugging.
Cleanup some error messages.
Introduce some errors into spatialdb file.
Small adjustments to errors.
Add more errors into mat_elastic and step02.
Finished setting up files with error and correct files. Moved correct files to correct directory.
Accidentally removed errors.
Added files to Makefile.
Added files to Makefile.
Fixed typo in comment.
Updated spline creation for compatibility with Trelis.