[tern.mmrm] Add new argument `residuals` to `g_mmrm_diagnostic()`

[danielinteractive]

Idea:

• allows to choose whether one wants 'raw' or 'standardized' residuals when using type = "fit-residual"
• this would be ignored when type = "q-q-residual" since then only 'standardized' residuals make sense
• refactoring of current function code might be valuable given length and complexity, although interface should stay (g_mmrm_diagnostic() for users)

Note: This came in from a user as a feature request.

[shajoezhu]

removing from sme backlog