Operand.toObject() should call Operand.convert() for list elements

[dbenn]

VeLaInterpreter.invokeJavaMethod() calls Operand.toObject() with lists but the latter does not call Operand.convert() for the list's elements. If any element does not have the required type, the actual parameter list building will fail, and so too the Java method invocation.

[dbenn]

For example, try this in VeLa:

x is [1.2 3.4 5]
y is [10.0 20.0 30.0 40.0]
scatter("" "" "" x y)

The scatter plot does not appear. Also, the associated error is swallowed.