pyhf: a pure Python implementation of HistFactory with tensors and autograd

The `HistFactory` p.d.f. template [[CERN-OPEN-2012-016](https://cds.cern.ch/record/1456844)] is per-se independent of its implementation in `ROOT` and it is useful to be able to run statistical analysis outside of the `ROOT`, `RooFit`, `RooStats` framework. `pyhf` is a pure-python implementation of that statistical model for multi-bin histogram-based analysis and its interval estimation is based on the asymptotic formulas of "Asymptotic formulae for likelihood-based tests of new physics" [[arxiv:1007.1727](https://arxiv.org/abs/1007.1727)]. `pyhf` supports modern computational graph libraries such as TensorFlow and PyTorch in order to make use of features such as autodifferentiation and GPU acceleration.