Throw error when calling `QGTOnTheFly.to_dense()` for non holomorphic ansatze

Created by: PhilipVinc

Apparently more than one year ago we had realised that QGTOnTheFly.to_dense returns numerically wrong results when the ansatz is non holomorphic and the parameters are complex (no problem for real parameters).

However, we had never implemented an error check, so sometimes it happened to me to try to run dynamics with QGTOnTheFly and non holomorphic, complex parameters ansatzes.

I want to put an end to this, so here this is.