Summation and dot product for efficiency

[rasmuse]

sum(fs.Variable() for i in range(n)) is slow for large n.
So is sum(fs.Variable() * parameter[i] for i in range(n)).

It should be simple to create another operator or two, called e.g. Sum and Dot, to allow for faster handling of such expressions.

[rasmuse]

Sum class implemented. Last changed in d10940b.

Sum is the most important of the two. Dot product should be straightforward to implement by copying and modifying Sum.