manyfeatures

WIP: adds lagrangian nn and simlple example

1

PR adds Lagrangian NN implementation (#352)  Evident drawbacks: - For Hessian and Jacobians all cross-derivatives are computed before extracting the useful ones - Only one-point data...

WIP: adds ltc layer and its example

9

PR adds Liquid Constant Time Layer [#509](https://github.com/SciML/DiffEqFlux.jl/issues/509#issue-830860839). Original [repo1](https://github.com/raminmh/liquid_time_constant_networks) and [repo2](https://github.com/mlech26l/keras-ncp) contain the code which is consistent with biological inspired NCP architecture.  The current implementation differs...