Bugfix: Allow compilation in C++98 mode

Bugfix: Allow compilation in C++98 mode

The boost variant boost::enable_if_c cannot be used to prevent the compiler
from trying to form Tensor<rank, dim, Number> objects with negative rank -
this results in an infinite template substitution and subsequent failure.

But we can use the tensor class itself as cheap "enable_if" mechanism -
just specialize Tensor<-1, dim> and Tensor<-2, dim> such that they do not
contain the necessary overloads.