Time dependent parameters/boundary condition

It is possible to define time dependent parameters by adding

time = variable(Constant(cell, count=2))

to the ufl file and then use 'time' in the definition of the parameter. The AST will contain a node variable node from a coefficient with count 2 (of cellwise constant value). (We reserve the value 2 for the time, just like 0 for ansatz and 1 for apply function).

In the visitor we check if the variable node has a Coefficient subnode with count 2 and we will just generate param.time in the c++ code (and tell loopy about it).