Slope Index of Inequality (SII) + calc_reliability

[HadleyBN]

Add in a new function for the slope index of inequality. Use the PHEIndicatorsMethods R package as a guide on how to write the function. Due to the complexity of this calculation, it will not be possible to directly translate what has been done in R into Python, so it will be quite challenging and time consuming to construct.

The calc_reliability() function in R is used as a wrapper function for additional functionality within the slope index of inequality function. I would suggest only writing this function AFTER writing the generic SII function so it is clearer where it might fit in and what the data might look like at that point within the pipeline/function.

Tests can be created inline with the tests in the PHEIndicatorsMethods R package.

The findxvalues() function is used in the SII function as part of the calculation.