Allow subtypes for general types

[m-mohr]

When choosing data types, there's an issue with some not being offered. For example, if string is allowed as type in general, the temporal strings should also be supported always as alternative.

[clausmichele]

Is this related? In apply_kernel, the data in the array is of type number, and complains about numbers with too many decimal digits:

[m-mohr]

Hmm, interesting. That looks more like a bug to me. I'll investigate. Which back-end is this? @clausmichele

Edit: I've fixed the bug in the code. As the Web Editor allows you to input arbitrary precision (I think) and just shows the error without any negative impact to the resulting process, I've not deployed yet.

[clausmichele]

Thanks! Yes, I've also noticed that it was only a warning and didn't affect the create process graph.