Make TensorProductElement n-ary

[Ivan Yashchuk]

Currently TensorProductElement accepts only two finite elements as arguments. Changing it to accept a list of finite elements will simplify the code to generate tensor product element of more than two elements.

[David Ham]

This is a fair point. However you probably don't want to use FIAT for multiway tensor product (maybe except at the very lowest polynomial degree) because the scaling of assembly using fully tabulated elements (i.e. what FIAT does) is absolutely awful. The solution to this is to use FInAT, which avoids the naïve tabulation and facilitates the form compiler generating the optimal sum factored assembly algorithm.