JPL 3.x Prolog-calls-Java gotchas

# JPL 3.x Prolog API gotchas

## calling methods with no parameters

> you must pass an empty parameter list when calling Java methods which take no parameters, e.g.

>     jpl_call('java.lang.System', gc, [], _)

## calling void methods

> you must accept an @(void) result when calling void Java methods, e.g. either

>     jpl_call('java.lang.System', gc, [], @(void))

> which explicitly matches the expected result, or

>     jpl_call('java.lang.System', gc, [], _)

> which uses an anonymous variable to ignore the result.

## (more to come...)