Template version of vector and Tensor

It allows to declare vectors (tensors) of arbitrary
size N (NxM). The difference wrt Matrix and std::vector
is that dimensions should be declared at compilation time.
This allows to save the storage of extra information
(useful for small 3 or 3x3 objects) and to optimize the code.

To improve the performance, I used a single n*m array
for Tensors (instead of a [n][m] array). However,
the external interface is untouched.

Finally, I defined some typedefs to simplify usage:
Vector (for 3d, so as to maintain compatibility with the rest of plumed)
Tensor (for 3x3d, so as to maintain compatibility with the rest of plumed)

From the tests that I performed, there is no visible overhead
when compared with the older (fixed 3 or 3x3) vector/tensor.