Updated math for correlated uncertainty ellipse

Updated the math for calculating the bezier approximations of
correlated uncertainty ellipses (called “error ellipses” in most of the
code) and added tests to ensure that the new method would be consistent
with the old one. The tests prove that the new method always produces a
matrix with numbers within 1e-14 of those in the matrix produced by the
old method, addressing concerns raised in #58