Cleanup: now that modules are reading from the database, there is no need for "singleton" modules anymore.
Updates to 2 files now that spreadmodel_read_prodtype returns just a single production type.
Minor: fixed missing "from" in SQL query.
Made the constructor for a piecewise distribution look more like the construction for a relational chart. Makes it easier to use the same SQL functions to set up both types of object.
Piecewise and histogram probability density functions are now read from the database.