Add support for filtering against null values in JEP

[Nicklas Nordborg]

It is not directly possible to filter out null (or non-null) values in a JEP filter. JEP filters are used by the FormulaFilter? plugin and in lots of other places.

A workaround is to use a filter that matches all values (except null), for example:

ch(1) > 0

ch(1) <= 0

This doesn't work if you want to keep the null values. One way is to add an isnull() function to JEP which is easy. Another idea is to simply be able to type 'ch(1) == null' or 'ch(1) != null' which might be trickier to implement.

[Nicklas Nordborg]

David Waring has submitted some code fixes on the mailing list. ​http://sourceforge.net/mailarchive/forum.php?thread_name=F18269A5-8ECC-48D8-9FB7-3371363AF249%40fhcrc.org&forum_name=basedb-devel

[Jari Häkkinen]

Milestone BASE 2.4 deleted

[Nicklas Nordborg]

(In [3456]) Fixes #564. It is now possible to use: ch(1) == null or ch(1) != null