Implement multi-line braille for multi-line braille display

Multiline "grid" braille displays have been introduced. They remain very expensive, but will undoubtedly get cheaper over time. Supporting them is an option for MathCAT. I believe that most braille codes have rules for line breaks and probably also rules for 2D layout of math. These rules are often similar to print layout rules, such as centering the numerator and denominator and drawing a "line" between them that is a little longer than either of them.

Each braille code has its own rules, although some (based on my memory) don't seem to be as fully developed as others.

Supporting multi-line display is definitely doable in MathCAT, but will be a larger project. I have opened this issue as a place to gather more information on what is needed for each braille code. Hopefully that helps to develop a good abstraction that can be shared among different codes.

This issue is also a place to gather interest from the community on priorities. For example, maybe 2D math is mainly important for tabular layouts like matrices and systems of equations. Or maybe those and fractions.

Comments are appreciated!