dynamicslab/pysindy: PDE-FIND and weak SINDy in arbitrary dimensions

This release allows for differentiation along specific axes, revamps the PDE-FIND and weak SINDy functionalities to work in arbitrary spatial dimensions, and moves all the array-reshaping to the internal code so the user-interface is easier. Finite difference coefficients are now computed on the fly depending on the derivative order "d" and accuracy order "order", i.e. finite differences now work for arbitrarily high derivative and accuracy order.

• See this Youtube playlist for tips on using pysindy in practice and examples of new functionality.

• Added a SpectralDerivative() class for spectral differentiation at faster speed than is available with the "derivative" python package.
• Finite Differences now have periodic boundary condition support.
• Cleaned up the example notebooks a bit.