Added RBF interpolation

• Added interpolation by RBF in RTree
• Improvement of RTree class performance

Binned statistics and bicubic interpolation in 3D space

• Addition of binned bivariate statistics.
• Addition of bicubic interpolation in 3D space.
• Improved generation of the C++ extension.
• Analysis of test coverage.
• Minor bug fixes.

Performance optimization

• Improvement of axis performance.
• Fixed a performance problem with bilinear, bicubic and trivariate interpolations.
• Handling of time axes in the xarray backend
• Access to main objects from the main module.

Use of the conda-forge channel

Using the conda-forge channel to install the software from anconda instead of the personal account.

Simplification of the Xarray backend

* Simplification of the Xarray backend

* Merging of the conda-forge recipe
* Fix documentation issues

Optimization of memory management

* Optimization of memory management

* Improving bicubic interpolation performance
* Fill NaN values.
* Major redesign to separate grid management from
  interpolation routines.