Version 0.7.0

This version removes deprecated functionality, and has several backend
improvements. The most noticeable user-facing change is the multithreaded fill
feature, which can enable significant speedups when you have a dataset that is
much larger than the number of bins in your histogram and have free cores to
use. Several small bugs have been fixed.

User changes

• Added threads= keyword to .fill and numpy functions; 0 for automatic, default is 1 #325
• .metadata is now settable directly from the AxesTuple #303
• Deprecated items from 0.5.x now dropped #301
• cpp mode updates and fixes #317

Bug fixes

• Dict indexing is now identical to positional indexing, fixes "picking" axes in dict #320
• Passing samples=None is now always allowed in .fill #325

Developer changes

• Build system update, higher requirements for developers (only) #314
  • Version is now obtained from setuptools_scm, no longer stored in repo
• Removed futures requirement for Python 2 tests
• Updated Boost.Histogram, cleaner code with fewer workarounds