Nicola Demo

copy from https://github.com/mathLab/PyDMD/pull/145

copy from https://github.com/mathLab/PyDMD/pull/186

"old" VTK files share the same extension `.vtk` but can contain different data type (polyData, UnstructuredGrid, ...). Such behaviour is currently not managed properly by the `VTKHandler` class

**Is your feature request related to a problem? Please describe.** Add in the [contributing file](https://github.com/mathLab/PINA/blob/master/CONTRIBUTING.md) information about: - [ ] how to run the tests locally - [ ] how...

I agree to divide conditions in multiple classes but I do not think that the `residual` method should be inside conditions. I see conditions more like an object that encapsulates...

I really like both `Optimizer` and `Scheduler` classes! I think the `hook` method can be made an abstract method for the base classes. In this way all the classes that...