mathjax 4 issue

1. overline issue
In MathJax3 `$\overline{GI}$` produce following mathml
```

```
which read as `line segment cap g cap i math`.

In MathJax4 `$\overline{GI}$` produce following mathml
```

```
which read as `quantity cap g of cap i with line above math`.

2. mhchem water issue
In mathJax3 `$\ce{H2O}$` produce following mathml
```

```
which read as `cap h sub 2 cap o math`.

In mathJax4 `$\ce{H2O}$` produce following mathml
```

No source file or test is named. Start by tracing MathCAT's handling of the invisible operators shown in the MathJax 3 and 4 MathML examples, then determine whether both inputs can be handled on the MathCAT side; done means establishing how the two cases should be processed.

Written by the indexing model from the issue text.