Integers with `Tensor::operator()`

[fredrikbk]

We would be able to use integers to index into Tensor::operator(). Ideally A(0,1) reads coordinate (0,1) and A(0,i) is an index expression that holds the first dimension fixed. (This might not work out; needs to be explored.)

So in addition to writing index expressions you can also use integers (and eventually a mix):

IndexVar i, j;
A(i,j) = B(i,j); // Index expression
A(0,1) = 1;      // Insertion
A(0,i);          // Read the first row?