Splinepy

This is the last version that creates wheels for python3.6 for PyPI distribution.
Here's a quick summery of the changes since the least release (v0.0.26):
- Pure python irit IO
- Volume integration
- Input/bounds/sanity checks to avoid segfaults
- Multipatch extractor extensions
- Switch from `TrackedArray` to `PhysicalSpaceArray`: instead of checking in-place changes of splines, we update properties right away to avoid creating new splines
- Pure python cats-xml IO
- Microstructure derivatives with respect to deformation function
- Change of numbering system of multipatch interfacces
- More show options for control mesh / control points
- mfem single-patch dof mapping for 2D and 3D
- Updated npz IO
- BSpline and NURBS supports arbitrary physical dimensions (previous only compiled up to 10)
- Bug/typo fixes.

In this version, all the extended functionalities that used to be implemented in `gustaf` is moved to `splinepy`. This adds `gustaf` as an additional dependency along with `numpy`. Here's a quick summary of the added features from `gustaf`:
- visualization
- (polygonal) mesh extraction
- free-form deformation
- create basic splines
- create new splines based on existing splines
- micro structures
- various micro tiles