Nurbs

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

* Bug fix: Use `six` package to maintain Python 2 and 3 interoperability for meta classes
* Bug fix: Updated `order` property for curves and `order_u`, `order_v`, `delta` properties for surfaces by removing excess conditional checks from their setters
* Updated error messages
* Updated documentation
* Updated tests
* Various minor updates and bug fixes

This is the official release of NURBS-Python (geomdl) v4.0.0. The following list summarizes the new and the updated features.

* Added [Plotly](https://plot.ly/python/) visualization module
* Improved algorithms
* Reorganized `exchange` module and added new export file types
* Export to file and no window options for the Visualization component
* NURBS module now allows setting control points and weights separately
* Load and save functionality
* New `Evaluator` module for changing evaluation algorithms at runtime
* New `convert` module for converting B-Spline objects to NURBS objects
* Improved the surface generator module
* Added more examples to the Examples repository: https://github.com/orbingol/NURBS-Python_Examples
* Documentation updates and improvements: http://nurbs-python.readthedocs.io/
* Bug fixes, code cleaning and compatibility updates
* New unit and function tests with [codecov.io](https://codecov.io/gh/orbingol/NURBS-Python) integration

* Fix a bug in surface generator causing miscalculation of the grid boundaries when users add padding via `base_adjust` argument
* Renamed surface control points row order changing functions (flipping)
* Updated smesh file importing API calls
* Minor updates in error messages and exceptions
* Documentation updates