Correction to null pointer in PogParamExpVisitor, fixes #303
updated test results po

this is mainly a list index change of the same result
updated string based po testing such that the following java property can be given to regenerate all tests:

-Dtests.po.override.all

or just

-Dtests.po.override.sl
-Dtests.po.override.pp
-Dtests.po.override.rt

note that results may change due to index changes in internal lists. This is safe to commit since it doesnt change the POs.
Record types for VDM-SL state should include an invariant, fixes #304
Fix problem with nil fields in sync clause expressions, fixes #302
Merge remote-tracking branch 'origin/ncb/development' into newpog
Update the old string pog test inputs and results.
Move POType and Status around to support old string based tests.
Added test classes for old string based tests.
Correct guards for static operations and variables
Correction to listener handling in value assignments
Prevent inheritance of private constructors, fixes #7
Merge commit '23b664c2306291160867c4b54c16730ab14018e3' into test
Merge remote-tracking branch 'origin/development' into newpog
Fixed whitespace cleaner (I hope).
Fixed error with construction of maplet set.
Corrected stored lexlocation.
Fixed type of map bind in finite map POs.