TST: add the Burkardt test matrices for the matrix exponential
MAINT: math.expm1 -> np.expm1 hoping for compatibility with old python/numpy
TST: clean up Burkardt expm matrix tests
TST: remove redundant code
TST: test eigenvalues instead of asserting them in comments
Merge pull request #2830 from argriffing/test-expm-burkardt

TST: add the Burkardt test matrices for the matrix exponential