Variable size separator groupings

[jagerber48]

Right now the upper and lower separators group the upper and lower digits into groups of three. It may be useful to group the digits into different sized groups. For example, it may be useful to group some binary representations into groups of 4. In fact, the add_group_chars function already has an input to control the number of grouping digits. This functionality is just not exposed to the user interface.

[jagerber48]

The possibility of variable sized separator groups could throw wrenches into input string parsing.

[jagerber48]

This was kind of going to be useful for binary representations, but actually, before #201, the binary modes were actually base-2 scientific formatting, not binary representations. Closing this since it is not useful.