Use kwargs in asm for additional fields

[kinnala]

As proposed in #360 (comment) .

After all it seemed to work fine. So the principle is now that

y = basis.interpolate(x)  # type: DiscreteField

and this gets passed to asm as keyword arguments:

asm(bilinf, basis, test=y)

This assumes e.g.

@BilinearForm
def bilinf(u, v, w):
    return u * v * w.test

Makes sense?

[kinnala]

I'll add some additional tests for confirming that it works with multiple fields later today.