Root-numpy

What's new in this release:

* coverage fixes and new build matrix for ROOT for CI
* support python 3.7 (with new cython)
* fix typos in docs

What's new in this release:

* silence cling errors in nosetests
* documentation improvements
* ignore duplicate keys with different cycles in list_objects
* additional improvements related to a code review for the JOSS paper submission

What's new in this release:
- The tests were missing from 4.7.1 and have been re-included

What's new in this release:
- Switched from MIT to new BSD license
- The branches argument for tree2array and root2array can contain tuples of the form `(branch_or_expression, fill_value, length)` that allow you to truncate variable-length branches or expressions at a fixed length or single value (if `length==1` or omitted) while imputing missing values where the original subarray was shorter than `length` with `fill_value`.

What's new in this release:
- Make numpy a requirement only if not already installed
- Support new TMVA DataLoader API
- hist2array: `return_edges=True` returns the bin edges along each axis
- stretch: if fields is single string (not a list) then flatten output
- tree2array and root2array: added new object_selection argument to perform selection on variable-length subarrays

Bugfixes:
- Fix rec2array shape consistency for subarrays
- Reset branch addresses after conversion in array2tree

What's new in this release:

tree2array:
- Enable tree caching and add cache_size argument
- Fix integer overflow in handling of trees with more than 2^31 entries
- More informative MemoryError if array is too large to fit in memory
- Consider start and stop arguments in initial array allocation to avoid filling memory when reading a large tree in chunks