[FEATURE REQUEST] append and vstack

[kwagyeman]

Describe the solution you'd like I would like if append() and vstack() could be implemented.

Also if this limitation could be removed.

Indices can also be a list of Booleans. By using a Boolean list, we can select those elements of an array that satisfy a specific condition. At the moment, such indexing is defined for row vectors only; when the rank of the tensor is higher than 1, the function raises a NotImplementedError exception, though this will be rectified in a future version of ulab.

Additional context I was trying to implement non-max-suppression using Ulab and ran into the lack of these features complicating things for me.

Happy to finance this work.