Solve (fractional) Fokker-Plank equation in TT-representation

[daskol]

Main Contributions

• Add tt.interpolate for common routines (Chebyshev and splines) for interpolation in TT-format.
• Implement Fokker-Plank equation solver in ttpy based on original implementation fpcross in teneva.
• [WIP] Implement fractional Fokker-Plank equation solver.
• Adjust core types (i.e. tt.core.vector and tt.core.matrix) for better interfacing with numpy.ndarray.

Auxiliary Contributions

• Introduce pytest and rework examples and existing tests to compliance with pytest.
• Move documentation generation to the repo root.
• Fix some linting errors.
• Remove excessive and trailing spaces almost everywhere.

[AutomaticHourglass]

This looks like very extensive work, why isn't this merged yet?

[daskol]

@AutomaticHourglass It is under publication review now.