Check for xdrop extensions of length 0

came up in #291
Merge pull request #292 from Garonenur/xdrop_bugs

Check for xdrop extensions of length 0
add getter fkt and tests
Closes #293.
Merge pull request #295 from standage/master

Closes #293.
Merge pull request #294 from Garonenur/intset

add getter fkt and tests to intset classes