Uproot3

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

Remove README from data_files

Includes PR 32: fixes `TH2.edges` and adds `TH2.allnumpy`.

Fixed 24, 27, 28, and 30.

Cleaning up the histogram interface to be more mature.

Minimum version for awkward is now 0.7.0.

Histograms now return Numpy arrays instead of lists of numbers for `values`, `allvalues`, `variances`, `allvariances`.

Added `edges`, `alledges`, `bins`, and `allbins`.