Add custom control structures

[mdbartos]

Taking pumps as an example, analogous functions for the following are needed:

operator_flow_coefficients https://github.com/mdbartos/pipedream/blob/0fc0159c4d21ec531d55b6cbedaa55ddc2e85ff5/pipedream_solver/nsuperlink.py#L978-L1009

solve_operator_flows https://github.com/mdbartos/pipedream/blob/0fc0159c4d21ec531d55b6cbedaa55ddc2e85ff5/pipedream_solver/nsuperlink.py#L1468-L1490

In sparse_matrix_equations, analogous lines for the following are needed:

https://github.com/mdbartos/pipedream/blob/0fc0159c4d21ec531d55b6cbedaa55ddc2e85ff5/pipedream_solver/nsuperlink.py#L1067-L1078

https://github.com/mdbartos/pipedream/blob/0fc0159c4d21ec531d55b6cbedaa55ddc2e85ff5/pipedream_solver/nsuperlink.py#L1145-L1159

In solve_sparse_matrix, a new matrix must be added to the sum:

https://github.com/mdbartos/pipedream/blob/0fc0159c4d21ec531d55b6cbedaa55ddc2e85ff5/pipedream_solver/nsuperlink.py#L1198

And same goes for solve_banded_matrix:

https://github.com/mdbartos/pipedream/blob/0fc0159c4d21ec531d55b6cbedaa55ddc2e85ff5/pipedream_solver/nsuperlink.py#L1243