Refactor Tensor class

[KAClough]

As a minimum we should do:

Make amrex::Real the default value of the data_t template argument for the Tensor class template i.e.

template <int rank, class data_t = amrex::Real, int size = DEFAULT_TENSOR_DIM> class Tensor

We can then remove amrex::Real when declaring a Tensor object e.g. Tensor<2> instead of Tensor<2, amrex::Real>.

But this is probably best done when we decide if we want to reformat it as a 1d array not a rank-d array, as suggested by @julianakwan.