Implement sumtensor class

[dmdunla]

%SUMTENSOR Class for implicit sum of other tensors.
%
%SUMTENSOR Methods:
% disp - Command window display of a sumtensor.
% display - Command window display of a sumtensor.
% double - Convert sumtensor to double array.
% full - Convert a sumtensor to a (dense) tensor.
% innerprod - Efficient inner product with a sumtensor.
% isscalar - False for sumtensors.
% mttkrp - Matricized tensor times Khatri-Rao product for sumtensor.
% ndims - Return the number of dimensions for a sumtensor.
% norm - Frobenius norm of a sumtensor.
% plus - Plus for sumtensor.
% size - Size of a sumtensor.
% subsref - Subscript reference for sumtensor.
% sumtensor - Tensor stored as sum of tensors.
% ttv - Tensor times vector for sumtensor.
% uminus - Unary minus for sumtensor.
% uplus - Unary plus for sumtensor.