Roc curve weights + update (#525)

* Add support for event weights in ROC calculations

* Update authors for ROCCurve

* Make TMVA factory use event weights

* Update authors of TMVA Factory

* Fix error in implementation of ROCCalc::ComputeSensitivity.

Also fixes axis labels and drawing direction to plot specificity on
x-axis and sensitivity on y-axis as is one standard.

* Fix ROC curve oscilation bug

Due to interpolation setting the ROC curve could sometimes be
non-monotonically decreasing. Changed to linear interpolation.

* Add ROC constructor for separate signal and background vectors

* Remove caching from ROCCurve::GetROCCurve

This since we might want to recalculate it with different
number of divisions.

* Fix ROC Curve crash when num_points < 2

* Format for clang-format

* Changes as discussed with L. Moneta

- Avoid unnecessary array copy
- Use reserve() to preallocate size of event arrays
- Use [] indexing instead of .at() since we assert sizes
- Use Double_t instead of Float_t for integral calc
- Flip axes of ROC plot

* Add documentation for ROCCurve public methods

* Fixes for clang-format

* More clang-format

* Last time with the doc clang-format