Remove usage of meshmode.dof_array.flatten and friends (#118)

* remove usage of meshmode.dof_array.flatten

* fix some flattening issues

* bump requirements to fixed branches

* fix some flake8 errors

* update unflatten usage

* port examples away from meshmode.dof_array.flatten

* port tests away from meshmode.dof_array.flatten

* fix some failing examples

* fix bug introduced in reordering sources

* fix some more unflatten mismatches

* use the new leaf_class argument to flatten

* point requirements to arraycontext branch

* fix issues from using leaf_class

* redirect requirements.txt to their rightful place