warn, don't stop, on missing observations

- fixes #20
minor changes in implementations of dss functions for readability and debugability
update NEWS
explicitly enforce numeric group ID
rewrite transformation of weights using sparse matrices and a home-grown Moore-Penrose inverse for our (very special) transformation matrix

- fixes #17
return correct weights -- regression introduced in 42dab3ff18999199dc3b (v0.0.9)
Bump version to 0.0-11
explicitly import methods to shut up R CMD check
update NEWS
normalize weights to get rid of precision problems
update NEWS
Merge branch 'master' into 10-slack