RooNDKeysPdf has no default constructor which causes a crash when reading it from a workspace

Hi,

while importing a RooNDKeysPdf into a Workspace you get the following message:
Warning in <TBufferFile::WriteObjectAny>: since RooNDKeysPdf has no public constructor
which can be called without argument, objects of this class
can not be read with the current library. You will need to
add a default constructor before attempting to read it.

And indeed, trying to recover the pdf from the workspace fails.
Storing it as a TObject might be a temporary workaround.

regards,
Nils