Uproot-methods

PR 47: Give TLorentzVectorArray, TVector3Array, TVector2Array `__awkward_persist__` methods so that they can be saved to and read from files.

Added type checks to `TLorentzVector*`, `TVector3*`, `TVector2*` constructor arguments to prevent common mistakes.

Also fixed a bug related that prevented TLorentzVectorArray + TLorentzVectorArray in some cases. (Nested within a JaggedArray, the class identity of TLorentzVectorArray sometimes wasn't carried through.)

Merged PR 42, which implements TH3.

It also introduces the following **breaking changes** in TH2:

* `xedges`, `yedges` are swapped (to their correct order)
* `.numpy()` returns a nested structure, like `numpy.histogramdd`, unlike `numpy.histogram2d`:

python
bin_content, (xedges, yedges) = th2.numpy()


for consistency with

python
bin_content, (xedges, yedges, zedges) = th3.numpy()

lgray introduced `PtEtaPhiMassLorentzVector` and `PtEtaPhiMassLorentzVectorArray`, which internally represent Lorentz vectors in cylindrical/kinematic coordinates, rather than cartesian. This avoids unnecessary conversions to cartesian if your data source is cylindrical/kinematic and you never need cartesian coordinates.

Also, guitargeek fixed a bug in TLorentzVector caching for cases in which `array.content != array.flatten()` (non-trivial physical structure for a given logical structure of jagged array).

Strings are now recognized for writing to ROOT files in Python 2 as well.

Now requires awkward 0.8.0 and all access to the awkward library goes through `ROOTMethods.awkward`.