Refactoring lagrange.jl

[sbadrian]

The lagrange.jl file has become quite lengthy. Would it make sense to

1. move it to global/
2. use lagrange.jl as main file and create subfiles for discontinuous and continuous lagrange functions? Potentially also moving the corresponding dual functions into a separate file (analogously to bcspace.jl)

Lastly, we might want to contemplate if we want to handle arbitrary interpolation points (not only equispaced). It is not a critical feature as in practice only the lowest orders are used, but at some point I would expect the Runge's phenomenon to be noticeable. I would think that with some trait this could be dealt with.