BUG: add proper error handling when using interp2d on regular grids

fixes #1889
BUG: fix test for interp2d
BLD: add numpy requirement to metadata if it can't be imported.

This should help Buildout users.  Closes gh-3379.
Corrected the theilslopes implementation and added a test
BUG: sparse: fix operator overloading with custom types

See issue #3580.
ENH: sparse: inplace ops should fall back to x = x op y if not implemented
MAINT: remove most references to symeig and move lobpcg benchmarks out of the test directory
MAINT: remove the rest of the symeig references
DOC: move docs from __init__.py to the lobpcg docstring
MAINT: avoid clobbering numpy.ma as ma
MAINT: change variable name 'iter' to 'nit' -- these names are used interchangeably in the code, but 'iter' is a python built-in function
MAINT: versionadded 0.14.0 -> 0.15.0
MAINT: avoid backslash for line continuation
STY: docstrings lines over 80 characters
STY: if not x is None -> if x is not None
TST: add a network flow test
TST: numpy.testing assertions already report the actual and desired values; also use a more sophisticated warning assertion
TST: test network flow with edges that have costs and limited capacities
TST: add the wikipedia simplex algorithm example as a test
TST: do not use zip() for bounds, because python3 can't find its len()