Added Airy2DKernel to testing and changed import order.
Added more kernel tests.
Changed tests to assert_all_close
More explicit error messages in core.py
Removed redundant odd/even check in Trapezoid kernels.
Added dicretize modes test.
Changed normalization test which failed on Travis due to numerical issues.
Merge pull request #1661 from adonath/Improve_coverage_convolution

Improve coverage convolution kernels