Nurbs

* Improved `linalg` module
* Added `control_points` module for managing control points
* Bug fixes and improvements
* Documentation updates

* Reduced set control points restrictions for surfaces. `Surface.set_ctrlpts` now accepts 2-dimensional control points.
* Deprecated `save` and `load` methods defined in `BSpline` module in favor of `import_json` and `export_json`
* Generalized `operations.add_dimension` API call to work with all `SplineGeometry` subclasses
* Documentation updates

* Fix incorrect dict export of trims in trimmed surfaces
* Add analytic trim curve support to trimmed surface dict export
* Change `VolumeContainer` base class from `SurfaceContainer` to `AbstractContainer` and add missing methods
* Documentation updates
* Minor bug fixes and improvements

* Convert "sense" to "reversed" for better understanding of trim curve orientation (API change for surface trimming)
* Update curve degree elevation and reduction algorithms and operations API (no API change)
* Fix trim curve exporting in `exchange.export_json`
* Deprecate `exchange.export_3dm` and `exchange.import_3dm` in favor of [rw3dm v2.0](https://github.com/orbingol/rw3dm)
* Documentation updates

* Minor refactoring in `elements` module (no API change)
* Visualization figure objects are now accessible outside the geometry classes, `render` method returns the figure object
* `vertex_spacing` is set to 1 for consistency between the exchange and visualization modules
* `vertices` property of the `Surface` class returns the exact list of vertices used in generation of the triangles
* Minor bug fixes
* Documentation updates

* Moved some `six` module functions and `backports_functools_lru_cache` module under *geomdl* directory
* Removed `typing` dependency (no time to add all typing hints, see docs for input and output types)
* Refactored imports, minimized internal dependencies
* Code rearrangement for some internal functions
* Refactored `operations` module (no API change)
* Updated `multi` module
* Updated `elements` module to make it consistent between its classes (minor API change)
* Improved dict-type file formats (JSON, YAML, CFG) to support trim curves and more (backwards compatible)
* Updated `exchange_vtk.export_polydata` to support tessellated surfaces and geometry containers
* Refactored `tessellation` module (internal API change, user API is the same and comes with improvements)
* Added `tessellate.QuadTessellate` class
* Added `==` and `!=`operators to `SplineGeometry` classes
* Added `freeform` module
* Improved the surface trimming algorithm to handle edge cases
* Added `trimming` module to fix different types of trim curves (open, multi-curve, etc.)
* Added an option to set trim curve sense; i.e. `sense = 0` means trim inside, `sense = 1` means trim outside
* Improved VTK visualization module to better handle surface and volume plots
* Added voxel visualization option to VTK visualization module
* Registered mode keypress events to VTK visualization module
* Added an option to generate custom visualization configuration while generating the visualization object
* General bug fixes and performance improvements
* Added more tests
* Documentation updates