Make demo and demo/meshes packages so they can imported in tests
adv_diff demo: add option to return the result for testing
Implement adv_diff regression test in pytest
Modularize laplace_ffc demo
laplace_ffc demo: add option to return the result for testing
Implement laplace_ffc regression test using pytest
Modularize mass2d_ffc demo
mass2d_ffc demo: add option to return the result for testing
Implement mass2d_ffc regression test using pytest
Modularize mass2d_triangle demo
mass2d_triangle demo: add option to return the result for testing
Add square.poly to regression test meshes dir
No more need to create meshes dir for regression tests
Implement mass2d_triangle regression test using pytest
Modularize mass_vector_ffc demo
mass_vector_ffc demo: add option to return the result for testing
Implement mass_vector_ffc regression test using pytest
Modularize weak_bcs_ffc demo
weak_bcs_ffc demo: add option to return the result for testing
Implement weak_bcs_ffc regression test using pytest