Almost done with the core functionality!

TO-DO

FAST-FOURIER TRANSFORM
hook up to either fftw (preferred) or use base::fft

BLOCK-CIRCULANT MATRICES
Explicitly construct BCM from a tensor with given face

Introduce column space unfold/fold (wrapper around general unfold/fold)

Fix all references to m_unfold/fold to row space unfold/fold in cp_als,
tucker_als, and mpca

???Define new tensor multiplication, transpose, orthogonality,
identity, range (maybe only include in the context of the decomposition)

Write order-3 fft_svd decomposition, using the t-svd algorithm as
provided in paper