Prevents dimension dropping when removing lp__

If there is only one parameter other than lp__, subsetting the matrix
turns it into a vector with no dimensions, thus causing ggs_chain to
fail when assessing the number of samples.

Using drop=FALSE ensures that the matrix stays a matrix, even if it has
only one column.
Merge pull request #20 from jucor/convertFromStan

Fix: issue #18 simple STAN test case