Rewrite as_scalar_pde using syzygy modules (#22)

* Rewrite as_scalar_pde using syzygy modules

* remove unnecessary diff

* use grevlex

* Address review comments

* Add blank line to pacify Flake8

* rename convert to matrix function

* remove * in generators

* syzygy -> ideal, dmp -> poly

* Use list comprehension

* Add types for as_scalar_pde and _get_all_scalar_pdes

* move docstring to public method

* Fix display math

* highlight EXCEPT

* Improve as_scalar_pde docs

* use a comprehension

* clarify None

* use min instead of loop

* need to go back to sympy exprs as polys in Matrices don't work

* Fix formatting

* need a simplify, otherwise we get 2*lam/(lam + 2*mu) + 4*mu/(lam + 2*mu) != 2

* use itertools accumulate

* do not assign a lambda expression

* Remove typo

* Fix _get_all_scalar_pdes type annotation

* Use math i

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* vec_idx -> comp_idx

Co-authored-by: Andreas Klöckner <inform@tiker.net>

* import List

* Link sympy issue

* Minor fixes to as_scalar_pde docstring

Co-authored-by: Andreas Klöckner <inform@tiker.net>