Class Definition in R, marray, etc.

Excluded Boost Multi_array to be possibly used, since it does not allow
for the number of dimensions to be indetermined at compile-time.

Found marray.hxx (see demo), which allows for just that - variable
number of parameters at compile-time.

Decided to design the class structure in R, and simply port speed
bottlenecks into C++. Use Rcpp to interface between R and C++, in
particular, RcppArmadillo to do matrix operations, since any tensor
operation can be decomposed into a series of matrix operations.

Created R virtual "Tensor" class, from which all other classes will
inherit. Right now the only real class is "ndTensor", with nd standing
for "numeric dense". Planned to implement in the future: "logical
dense", "integer dense", "logical sparse", "integer sparse", "numeric
sparse". Should be simple once ndTensor is completed set up. So right
now just work with ndTensor as the core class.

Created "ndTensor" class that inherits from "Tensor", and created the
default constructor.

Created as.tensor() function to provide user-friendly set up of a
tensor from vectors, matrices, and arrays. Other types are not allowed.

Next on the agenda: implement print() and show() methods, add/subtract
methods, tensor_times_vector(), tensor_time_matrix(), matricize(),
unmatricize(). Also need to overload some operators in R to make
operations user-friendly.