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
BUG: sparse: itruediv should be allowed on sparse matrix
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
MAINT: change variable names that used incorrect terminology
MAINT: remove the '__main__' section and the X.txt file from lobpcg
TST: the '__main__' lobpcg.py code is now a real test
Adds default params to pdfs of multivariate_norm

Adds default params to pdf and logpdf to match
`norm` and the documentation.
BUG: signal.welch: ensure floating point dtype regardless of input dtype

Fixes gh-3655.
DOC: adding differential_evolution function to 0.15 release notes
Merge pull request #3671 from andyfaff/diffev-releasenotes

DOC: adding differential_evolution function to 0.15 release notes
Merge pull request #3599 from argriffing/remove-symeig

MAINT: symeig removal
Merge pull request #3667 from tshauck/multivariate-norm-docs

ENH: Adds default params to pdfs of multivariate_norm
Merge pull request #3657 from ewmoore/specint

BUG: signal.welch: ensure floating point dtype regardless of input dtype