fix subMgsaSets()

- items parameter is a character vector of ids
- original item ids are preserved after subsetting
- item and set annotations are preserved
enhance getting item/set annotations

- require exact name match
- always return data frame
add tests infrastructure using testthat package
add subMgsaSets test
draw lines in parameter distribution plots
Merge branch 'fix-subset' of https://github.com/alyst/mgsa-bioc
Suggest the testthat package.