Merge branch 'jed/ceed-error-exit' [PR #126]

* jed/ceed-error-exit:
Add CeedErrorExit to exit without signals, use in tests

Add CeedErrorExit to exit without signals, use in tests

This allows gcov to finish writing outputs even if the test
crashes (especially in tests that confirm error handling).

Merge branch 'cleanup' [PR #118]

* cleanup:
make style, excluding backends/{occa,magma}
make style: fix interface and include paths
docs: fix capitalization
doc: add developer notes on shape

Merge branch 'cleanup' [PR #118]

* cleanup:
make style, excluding backends/{occa,magma}
make style: fix interface and include paths
docs: fix capitalization
doc: add developer notes on shape and adopt convention
Standardize CeedIntPow and CeedIntMin
Move and document CeedIntMin, document CeedPowInt
Add function levels
Update Doxygen output naming
Add Test List to Doxygen
Doxygen interface comment updates
Remove redundant doxygen comments
Documentation updating for t500
Move ceed* files to 'inteface' directory, comment cleanup
Further CPU backend commenting and cleaning
Reorder tests, renumber for future expansion
Clean up and tighten Opt and Ref backends

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Add function levels

Doxygen interface comment updates

Move ceed* files to 'inteface' directory, comment cleanup