Generalise the representation of OTF's constraints
to match the new dolfin-adjoint framework.

This temporarily breaks examples/orkney_steady (SLSQP doesn't
handle multiple constraints correctly, I think), but I'll look
at that later.
Update solver params
Use SLSQP from pyOpt. This appears to work (or at least run)
Tweak the slsqp settings.
Merge pull request #1 from pefarrell/generalise-constraints

Generalise the representation of OTF's constraints