Tests facility

After the pre-alpha development we need to design a suitable tests facilities. The library is aimed to be a general approximation (interpolation) tool thus the range of applications is quite wide. Nevertheless, one absolutely necessary test is its usage into the non linear PDEs solving framework. Due to my background, I will add a very simple 1D Riemann solver of compressible Euler PDEs system for testing the library: this should be simple enough to be a yet clear test while it retains a lot complexity (unsteady non linear hyperbolic system).

Here I would like to discuss and list all possible tests.
### Tests list
- [ ] 1D Riemann solver for the compressible Euler system;
- [ ] Shu-Osher problem;
- [ ] image interpolation (procedure for increasing resolution)?
- [ ] check conservative feature of the interpolations when coupled with (fully) conservative schemes;
- [x] reconstruct the `sin` trigonometric fuction;