Change license to a permissive licence

[uvchik]

The windpowerlib is a basic library that should be used in other libraries. The GPL3 makes it more complicate to use it in projects with permissive licenses such as BSD, MIT, Apache...

Therefore, I would like to change the license to a more permissive license.

The easy MIT licences could be a good choice.

[uvchik]

I started the same discussion in oemof/organisation. We can discuss it there for the whole framework and follow the decission if no one objects.

[SabineHaas]

I have merged your PR #77 - thank you!