Introduce equationcd as shortcut for equation + tikzcd environment

[tobiasdiez]

It is quite common to display commutative diagrams as display equations (in fact, I would argue that 90% of all diagrams are displayed in this way). For this one has to wrap the tikzcd environment in an equation environment, which leads to a quite complex construction.

This PR introduces a new environment equationcd that wraps tikzcd in equation, so that one can simply write

\begin{equationcd][label = equationLabel, tikz = tikzcdStyles]
commutative diagram comes here
\end{equationcd}

Things to do:

• [ ] Document addition

[astoff]

I'm a bit conservative at this point about adding smart functionality like this because it can hard to get all corner cases working right. That said:

• The package must be format-independent (like TikZ itself) and not depend on expl3
• The syntax \begin{equationcd}[some regular tikzcd option], as opposed to \begin{equationcd}[tikz = some regular tikzcd option], must work. Especially given that if commas or equal signs are involved, you need to "protect" everything with braces, which is annoying.

I also think the benefits of this feature has to be weighted against the simplicity of leaving it to the user to define, say, \newenvironment{equationcd*}{\[\tikzcd}{\endtikzcd\]}, which covers at least the unnumbered version.