Lazy semantics for compile/assemble/compute in Tensors

[fredrikbk]

The current state machine semantics for TensorBase::operator(), TensorBase::compile, TensorBase::compute and TensorBase::assemble can be difficult to get right. They can also cause code to be less clear since you have to build and compile expressions outside the loop where you want to do the computation:

a(i) = B(i,j)*c(j);
a.compile();
a.assemble();
while (!converged(a)) {
  a.compute();
}

We should consider lazy semantics, where you can write:

while (!converged(a)) {
  a(i) = B(i,j)*c(j);
}