Added new spiral part where length can be set

[fbeutel]

This replaces the old spiral with a new version of an archimedean spiral which allows to set the length. The number of turns required to achieve the length is determined numerically (afaik an analytical solution doesn't exist).

Different types are supported. Especially for use inside an MZI the inline and inline_rel types are of interest.

ToDo's before merge:

• Update changelog
• Add to tests
• Update documentation
• Allow choosing the spiral direction (752d3b2)
• Choose an appropriate guess value for the numerical estimation

[HelgeGehring]

Nice Spiral!

• Could you remove the (object) in the line where you define the class? It was only necessary in python 2 (see link)

About backward compatibility: I'd guess it would be good to first deprecate the old spiral and have both for one version. Afterwards, we remove the old one and then also replace the name?

[fbeutel]

Yeah, about backwards compatibility I'm also not sure. One could try to make it backwards compatible to the old spiral, but I think that would be a rather ugly hack, so maybe it's better to go with two spirals for now and replace the old one later on...

[HelgeGehring]

No, I think finally the spiral should just be replaced, so please no ugly hacks ;)
But for at least one version we should deprecate the old one and offer both. For the following release we then can remove the old spiral I'd say.
Besides that: I'd guess it would be a good idea to let all functions you define in the beginning start with an underline in order to keep the interface simple.

[fengsuchun]

Could you add squared spiral with euler bend？

[fbeutel]

@HelgeGehring Can you add the changelog entry as you see fit? Other than that I think it's ready for review