boundary conditions in time

Hi,

I have just pushed in a new functionality concerning boundary conditions.

Now it is possible to specify times in which a particular condition is active, which enables not only changing the value of a condition, but also the region where it applies.

Imagine applying a given dislacement to various parts of a specimen in the course of time.

See also the new examples/diffusion/laplace_time_ebcs.py example.

r.