Bugfix in lookup_1d_functiontable (tables with just one point).
lookup_1d_functiontable now handles the case of a table with a single point (xp,yp) correctly. It then returns the associated yp for any value of x. Before this change, it returned NAN for x!=xp and yp for x==xp. Additionally, the functions lookup_1d_functiontable and lookup_2d_functiontable now return NAN (not a number) in case the table doesn't contain any values.