Add transform keyword to generic attach method (second try)

[pablosanjose]

This adds transform to attach(::GreenSlice, ::AbstractModel; transform = ..., sites...)

We also check against empty contact selections, which formerly threw an obscure error. Now the error is more explicit.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.48%. Comparing base (4276162) to head (ca5253e).

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #263   +/-   ##
=======================================
  Coverage   92.48%   92.48%           
=======================================
  Files          36       36           
  Lines        5880     5882    +2     
=======================================
+ Hits         5438     5440    +2     
  Misses        442      442           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.