Extended Gettable javadoc comments

The getValue method is expected to return a null value for an invalid
parameter id. The javadoc comments have been updated specify this.
Cleaned up a test case

InPUTConfigTest:
Replaced an if-clause followed by an explicit call to fail() with a call to
assertTrue.
Cleaned up a test in DesignSpaceTest.java

testDesignSpaceWithMappingAsInputStreams:
The test method is declared to throw an InPUTException and a
FileNotFoundException. No exceptions are caught.