Add support for optional parameters for linearRegression

[npazosmendez]

Is your feature request related to a problem? Please describe.

The official docs say linearRegression supports two optional parameters that change the range of the linear regression: linearRegression(seriesList, startSourceAt=None, endSourceAt=None). See here. CarbonAPI does not support these. It uses the from/to from the input series and just fills out the NaNs.

Describe the solution you'd like

The startSourceAt and endSourceAt should be used if passed to shift the ranges of the linear regression.

Additional context
We already tried to add support for this #40 but realized it requires more knowledge on how the regression is computed. And since it's not a priority for us right now, we are creating this issue for later.

[npazosmendez]

@carrieedwards was this one addressed? I can't find a PR either