Nurbs

* The core library now utilizes `Abstract` and `Multi` modules
* Curve classes now have `split()` method for curve splitting at the given parameter and `decompose()` method for Bézier decomposition
* Shapes module upgrades

* Added `decompose` method to curve classes for applying Bézier decomposition
* Added `add_list` method to `Multi` module

* Minor bug fixes
* Added `translate()` function to curve classes

* Fix a control points copy error in `insert_knot()` methods
* Increased stability of `generate_knot_vector()` function
* Some other minor fixes in `exchange` module
* `save_obj` can save single or multiple surfaces to a single .obj file
* Curve and Curve2D classes have a new method, `split()`. This method splits the curve and returns 2 new instances as the split pieces of the initial curve. It doesn't modify the initial curve.
* `Multi` module is designed to operate on multiple curves and surfaces. Currently, it can only do multi curve and multi surface visualization (in 2D and 3D) on the same window. Documentation will come soon.
* `Abstract` module provides a base for further development of the NURBS algorithms. It will become the base class for B-Spline and NURBS curves and surfaces represented in NURBS-Python. Unfortunately, the module is still WIP and it could remain like that for a while.

* Performance improvements
* Documentation updates

This version comes with stability updates and more tests to cover the majority of the module functionality. The documentation is also improved.

* Added `exchange` module with the capability to save surfaces as .obj files
* Added `shapes` module to automatically generate well-defined NURBS curves and surfaces
* More configurable `visualization` module
* Bug and stability fixes
* Documentation updates