new sketch and addition to LoftedShape

[LasseHolch]

shape.py:
Possibility to create spline from multiple mid_sketches added.

spline_round.py:
Two new sketches added

[FranzBangar]

Wow, that was unexpected! Great!
Just a few thoughts before merge:

1. You're setting u_1 and other properties in __init__(). When a sketch is transformed, will these properties update or remain as in the beginning?
2. QuarterSplineRound and QuarterSplineRing have a lot of duplicated code. How much could be reused?
3. Also you're adding 11 and more attributes to your new classes. Not that I'm sticking blindly to the 'max-7-attributes-rule' but I guess some could be grouped into a separate class and reused in both of the above classes?
4. Can you add an example that demonstrates the usage of the two classes? I can update the documentation that shows blocking of these sketches.

Anyway, I'm really glad that someone has the will and eagerness to contribute! Thanks!

[FranzBangar]

Funny, while reviewing your contributions, I found out my own hollow shapes (rings) don't work with transforms. Please stand by while I untangle all this.

[FranzBangar]

Sorry, false alarm, everything seems to be in order, I mixed up traumas from the past.

Anyway, I managed to sort out most of the typing conundrums (except for the spline side edges). I replaced property setters (which generate tons of boilerplate code) and conformed to creation pattern in other sketches. Most of your code (including maths) remains the same. Please take look and let me know if you think that's okay.

Here's the commit: 0c6a59f

I'll solve the other issues next, then I'll merge it into master.
Thanks!

[FranzBangar]

Here it is. And finally, you're on the list of contributors!

[LasseHolch]

Thank you so much.