ExnerBedloadTerm does not work with generic boundary conditions

[tkarna]

At the moment the ExnerBedloadTerm checks whether zeros have been prescribed at the boundary to decide whether to include the boundary term or not:

thetis/thetis/exner_eq.py

Lines 115 to 127 in 8685cd9

	for bnd_marker in (bnd_conditions or []):
	no_contr = False
	keys = [*bnd_conditions[bnd_marker].keys()]
	values = [*bnd_conditions[bnd_marker].values()]
	for i in range(len(keys)):
	if keys[i] not in ('elev', 'uv'):
	if float(values[i]) == 0.0:
	no_contr = True
	elif keys[i] == 'uv':
	if all(j == 0.0 for j in [float(j) for j in values[i]]):
	no_contr = True
	if not no_contr:
	f += -self.test*(fac*qbx*self.n[0] + fac*qby*self.n[1])*self.ds(bnd_marker)

This test, however, does not work if the boundary values are UFL expressions for instance (as in the case of tides with a time-dependent ramp up). We should find a better way of imposing the boundaries.

Why the 0.0 is a special case here? If we need to manually turn off the bedload term at certain boundaries I think we could add a special boundary flag to handle it. Maybe @mc4117 has some ideas.