Research the dieresis situation

There's strange contradiction I even had to [describe in the documentation](https://github.com/ForNeVeR/wpf-math/pull/108/files#diff-2dc54592d7800db71c597c416c9a29abR94):

> How to know that `O 177` is the same as `code="196"`? To do that, first look into `cmmi10.vpl` file: there's the following entry:
>
> ```
> (CHARACTER O 177 (comment dieresis)
> (CHARWD R 583)
> (CHARHT R 705)
> (CHARIC R 118)
> (MAP
> (SETCHAR O 151)
> )
> )
> ```
>
> That means that `O 177` is named `dieresis`. Then, open [Adobe Glyph List][glyphlist] and search for the `dieresis` name:
>
> ```
> dieresis;00A8
> ```
>
> It means that `O 177` should be character `0xa8` or `168`, not `196`. The reason for that contradiction is currently unknown.

We need to research that: what does `code="196"` mean and what's the reason the diaresis isn't 168?

We need to find the answer and write in into the documentation.

[glyphlist]: https://github.com/adobe-type-tools/agl-aglfn/blob/5de337bfa018e480bf15b77973e27ccdbada8e56/glyphlist.txt