Topic/plat 6697

Added vector function [PLAT}-6697 and concrete implementations plus tests 
Whilst I understand the intent of this test (faking up 3 functions and concatenating them), this test class is not sufficient. Nothing is asserted and nothing significant is tested in that which is instantiated.

Please rename this class something like "ConcatenatedVectorFunctionTest". Then write tests with method names reflecting the functionality they are testing and then assert that the answers gained are those from theory. Remove printing to `stdout`. Please also add this class to `@Test(groups = TestGroup.UNIT)` so that the coverage tools/test frameworks pick up the class for their reports.