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

fixes #1889
BUG: fix test for interp2d
BUG: cluster.linkage array conversion to double dtype
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
Moved implementation to `stats.theilslopes`. Added tests to `stats`.
Added y ties term to the confidence interval calculation.
PEP8 fixes
BLD: add minimum required numpy version to build_requires if numpy not found.
BUG: sparse: itruediv should be allowed on sparse matrix
MAINT integrate/odepack: kill dead Fortran code
BENCH: lobpcg vs. eigh mikota pair benchmark now works
BENCH: let the preconditioner do mat-mat explicitly, not just mat-vec
BENCH: resurrect another lobpcg benchmark
MAINT: remove unused imports