collapse in tensor class errors, not well documented on what the format of dims is.

[jdtuck]

When using collapse method in the tensor class, the parameter dims is not clear and leads to errors.

[dmdunla]

collapse currently does not work fully, as it depends on the tenmat class, which is not yet implemented. A collapse across all dimensions works, but other subsets of dimensions will return an AssertionError until tenmat is implemented.