Vertically center single-character \mathop.

[kohler]

Fixes issue #726.

[kevinbarabash]

@kohler how did you test this? Could you post some (before/after) screenshots \mathop expressions that are affected by this change?

[kohler]

Sure, I tested many expressions. Here's an expression that encompasses a lot of em

a\mathop+b\mathop:c\mathop{\delta}e\mathop{\textrm{and}}f\mathrel{\mathop{:}}=g\sin h

Before

After

Note that the :s moved up (as they should), the \delta moved down (as it should: https://tex.stackexchange.com/questions/368673/documentation-of-mathop), and the other ones stayed put (as they should).

[kohler]

NB Other uses of groupTypes.op that could have been affected, such as \stackrel, are tested in the screenshotter tests.

[kevinbarabash]

Looks good. Can you add a screenshot test for that? Screenshots for single tests can be generated using the include option, e.g. dockers/Screenshotter/screenshotter.sh --include=Sqrt.

[kohler]

test added

[edemaine]

Looks great! Thanks for fixing this.