bfGetReader() unit tests (rebased onto develop)



This is the same as gh-464 but rebased onto develop.

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Summary of changes
- allow fake string to be passed to `bfGetReader()` for mock testing
- create unit tests for `bfGetReader()` and refactor `bfGetPlane()` unit tests

Can be tested using the Matlab test target ``MATLABPATH=path/to/xunit ant test-matlab``. All tests should pass.