Symfem

- Allow "gl" variant of Lagrange element
- Implement component-wise integrals of VectorFunction and MatrixFunction
- Implement grad of VectorFunction
- Add Gopalakrishnan-Lederer-Schoberl element
- Use polynomial subdegree to index elements wherever possible

- Corrected nonconforming Arnold-Winther polyset
- Correct Alfeld-Sorokina range dim
- Correct Guzman-Neilan second kind element
- Add Guzman-Neilan first kind element
- Correct Guzman-Neilan bubbles
- Add discontinuous elements

- Corrected polyset for Arnold-Winther element
- Added sub- and superdegree functions

- Added get_mapping function

- Link to images from correct version

- Fix readme on PyPI