degroup() for nested models

[strengejacke]

Fixes #530

[strengejacke]

@jmgirard you may "review" this PR, too, if you like.

[strengejacke]

@jmgirard do you think we can merge this? You may look just at the changes in demean.R and the related test file in this PR for a quick review... Then this PR could probably included in the next release.

[jmgirard]

@jmgirard do you think we can merge this? You may look just at the changes in demean.R and the related test file in this PR for a quick review... Then this PR could probably included in the next release.

The code and test looks good. The documentation should mention somewhere that a hybrid model that includes both nested and crossed levels is currently unsupported. I'm also still wondering if it is better to use names like {"math_between_school", "math_between_classroom", and "math_within"} or {"math_L3", "math_L2", and "math_L1"}. Perhaps the suffix_* arguments could be customized for nested models?

[strengejacke]

Using names instead of L3, L2 etc. follows the same scheme as for cross classified designs. Maybe we can ind ed extend the suffix argument to allow different naming schemes. But I would keep that for a later update.