add aggregation functions including corr, covar,regr-slope, regr-interce...

add aggregation functions including corr, covar,regr-slope, regr-intercept and unit tests.

Should we reuse the same AbstractTestAggregationFunction? since we cannot reuse the unit tests
for example: the unit tests in AbstractTestAggregationFunction:
    @Test
    public void testNoPositions()
    {
        testAggregation(getExpectedValue(0, 0), getSequenceBlock(0, 0));
    }

    @Test
    public void testSinglePosition()
    {
        testAggregation(getExpectedValue(0, 1), getSequenceBlock(0, 1));
    }

    @Test
    public void testMultiplePositions()
    {
        testAggregation(getExpectedValue(0, 5), getSequenceBlock(0, 5));
    }

the unit tests in AbstractTestCovarianceAggregationFunction
   @Test
    public void testNoPositions()
    {
        testAggregation(getExpectedValue(0, 0, 0), getDoubleSequenceBlock(0, 0), getDoubleSequenceBlock(0, 0));
    }

    @Test
    public void testSinglePosition()
    {
        testAggregation(getExpectedValue(0, 1, 1), getDoubleSequenceBlock(0, 1), getDoubleSequenceBlock(1, 1));
    }

    @Test
    public void testMultiplePositions()
    {
        testAggregation(getExpectedValue(0, 2, 5), getDoubleSequenceBlock(0, 5), getDoubleSequenceBlock(2, 5));
    }
even if we use AbstractTestAggregationFunction class, we still have to override those unit tests. Correct me if I am wrong. Thanks!