Allow for complex values states/operators/snapshot-matrices

[opaliss]

Is your feature request related to a problem? Please describe.
It would be great to allow for complex-valued states/operators/snapshot matrices.

Describe the solution you'd like
Allow integration of complex-valued ODEs + solve the least squares problem using complex-valued snapshot matrices. I believe this could be done easily by changing the dtype in certain definitions to allow complex128.

[shanemcq18]

This is a great idea. Ideally we should check the type of the states/inputs so we don't use a complex type unless necessary, but this shouldn't be too bad.

Places to check for updates:

Operators:

• set_entries()
• apply()
• jacobian()

Models:

• _process_fit_arguments()
• rhs()
• jacobian()
• predict()