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.