Put all Basis inside a src/basis directory

[orkolorko]

The actual directory structure of the package is somewhat confused and polluted; it is worth thinking on a directory structure, maybe as follows

src/basis/1D/Ulam.jl

At the moment the basis files are in the project root, and there are no standardized names.

[fph]

Seems a good idea to me. One separate directory for the bases, one for dynamic definitions and preimages, and one for related to the norm, dfly and assembler. The submodule structure could maybe also be simplified; for instance with one single module for each folder.