Add explicit GPLv3 license to the files depending on pygmsh

[kinnala]

Closes #445

[kinnala]

@gdmcbain and @ahhuhtal I suppose you're fine with this clarification?

[gdmcbain]

Yes.

As noted in #445, ex19 depends on scikit-sparse which is itself two-clause BSD as a wrapper, but the CHOLMOD that it wraps is GPL 2.1.

https://github.com/PetterS/SuiteSparse/blob/27e5a8516464a6ac40bd3fa0e5b46e51b11f4765/CHOLMOD/Cholesky/cholmod_factorize.c#L5-L10

How does that work? I don't know. I figure splu will be fine.

[kinnala]

I think scikit-sparse README explains the rationale behind the licensing in a reasonable manner:

(The intent here is that if you, for example, buy a license to use CHOLMOD in a commercial product, then you can also go ahead and use our wrapper code with your commercial license.)

If the wrapper code was GPL-licensed and used together with a commercial license of CHOLMOD then technically distributing a commercial product based on those two would still require disclosing the source code of the wrapper code or any modifications made to it.

In Python code, dynamic linking happens only after a user runs the code in a Python interpreter on his own computer and some people have interpreted that only this is the step where the GPL-rot will spread, and hence justify different licensing of code that imports GPL-components, basically pushing the responsibility of licensing downstream. I have personally taken a different, more strict stance on this and consider anything that imports GPL 'dangerous' unless also GPL licensed.

As you can see, the whole GPL thing is a bit of a mess and that's why I try to stay out of it as much as I can.