Numba sparse: handle dot product between matrices more granularly to reduce the number of of format conversions

* Add comment regarding meaning of '*' operator with sparse matrices

* Implement sparse dot product in numba

* Overload T attribute in numba sparse matrices

* Implement toarray

* Add tocsr

Co-authored-by: Jesse Grabowski <48652735+jessegrabowski@users.noreply.github.com>

Co-authored-by: Jesse Grabowski <48652735+jessegrabowski@users.noreply.github.com>

Co-authored-by: Adrian Seyboldt <aseyboldt@users.noreply.github.com>
Co-authored-by: Jesse Grabowski <48652735+jessegrabowski@users.noreply.github.com>

2304 -> 288

* Handle static shape
* Rename to more readable Op classes
* Simplify perform

Add Schur decomposition Op

Co-authored-by: Jesse Grabowski <48652735+jessegrabowski@users.noreply.github.com>

Co-authored-by: Jesse Grabowski <48652735+jessegrabowski@users.noreply.github.com>

* More precise imports

* Fix gradient of `minimize` and `root` wrt higher-dimensional arguments

Also:
* Allow several inputs to be optimized
* Handle disconnected and undefined gradients

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Co-authored-by: ricardoV94 <ricardo.vieira1994@gmail.com>