Added unit tests for ErrorEllipse.getControlPoints()

Added unit tests for ErrorEllipse.getControlPoints() to ensure that it
works correctly and to prevent future regressions.
Fixed issue #2

This commit fixes #2, in which the standalone application improperly
plotted error ellipses.

The bug was caused by a simple typo which caused sigma y to be passed
for sigma x.
Merge branch 'error-ellipse-fix' into dev