Remove `as_complex_array` in favor of `ComplexArray` object.

[s-mandra]

For performance reasons, simulate may use either a single array of complex numbers (when using optimize=evolution-einsum or backend='jax') or a pair or real valued arrays (when using optimize=evolution-hybridq). For this reasons, some functions require to specify as_complex_array to discern from the two cases. Using ComplexArray would remove the necessity to extra flags.