Symfem

- Added Huang-Zhang element
- Added adding an element guide
- Increased documentation

- Remove svgwrite dependency
- Added normalised orthogonal functions

- Added plotting of DOF diagrams
- Added P1-iso-P2 element
- Added typing
- Added plotting of basis functions
- Added plotting of entity numbering

- Added Rannacher-Turek element
- Added functions to create orthogonal polynomials on cells
- Added TeX descriptions of DOF functionals
- Corrected Bernstein element

- Added Regge element on tensor product cells.
- Added tensor product factorisation of Q element.
- Added Arnold-Boffi-Falk element
- Added Arbogast-Correa element

- Removed discontinuous elements.
- Added pictures of reference cells to readme.
- Added trimmed serendipity Hdiv and Hcurl elements.
- Added TNT scalar, Hdiv and Hcurl elements.