Added a convenience method to get an isotope based on an element symbol and exact mass, returning null when nothing suitable is found

Change-Id: Ia40b52db1f8daf31ec4d8197ef05e35270d199fd
Signed-off-by: John May <john.wilkinsonmay@gmail.com>
return the matching isotope closest to the provided exact mass

Change-Id: I842af6599bc25ebfab9b6f1f7a150973c8e253c3
Signed-off-by: Egon Willighagen <egonw@users.sourceforge.net>