Nurbs-python

* Added `Tessellate` class for customization of the surface tessellation algorithms
* Basic trimmed surface support (still work in progress)
* Updated `elements` module
* Visualization module improvements and added support for displaying trim curves for surfaces
* Added `evaluate_single` and `evaluate_list` methods
* Updated default evaluation delta to 0.01
* Minor bug fixes, algorithm and performance improvements

* Fixed a bug causing incorrect alignment of control points that generate the hills in the surface generator module
* Surface generator now generates more smooth hills
* Added an option to change the label on the visualization legend by setting the ``name`` property
* Updated ``Multi.MultiCurve`` and ``Multi.MultiSurface`` constructors to allow easy addition of the shapes to the container object

This release adds the functionality of finding control points involved in the evaluation of a curve/surface at the
specified parameter(s) using `operations.find_ctrlpts()`.

* Added `operations.find_ctrlpts()` function
* Minor improvements in the surface evaluation algorithm

This release moves some of the features from `BSpline` module to other modules since it is getting bigger and it has a lot of responsibilities which can be "globalized" to ease extensibility.

* Code reorganization and cleanup for Curve and Surface classes
* Removed plural-named methods, such as `tangents`, `normals`, etc and moved the functionality to singular methods (`normal`, `tangent`, etc.)
* Moved `translate`, `split` and `decompose` methods to `operations` module
* `derivatives` method is added as an abstract method to `Abstract.Curve` and `Abstract.Surface` since it is also a requirement for all `Evaluator` implementations.

The following new and updated features are added with this release:

* New module: `operations`, it contains geometric operations that can be applied to curves and surfaces
* Added libconfig-type file export feature to `exchange` module
* Updated triangulation functions
* Updated `elements` module
* Updated `VisMPL.VisSurfTriangle()` class to use `utilities.make_triangle_mesh()` for triangulation
* Added colormap input to `VisMPL.VisSurfTriangle()` class
* Fixed deprecation errors for Plotly >= 3.0.0
* Added new vector and point operations to `utilities` module

* Added algorithms A2.4, A2.5, A3.7 and A3.8
* Added an option to generate knot vectors for unclamped shapes
* Bug fixes and minor updates
* Documentation updates

* Bug fix: Fixed an inconsistent behavior while setting `delta` and `sample_size` properties