ENH: avoid repeated matrix inverse

The `sum_cov` term doesn't change, so it's faster to use a factorized version and solve.
ENH: Add ext parameter to UnivariateSpline call

Pass parameter to fitpack.splev to control how to handle out of range
evaluations. Added test to scipy/interpolate/tests/test_fitpack2.py.

See ticket 3557.

closes gh-3557
STY avoid unnecessary transposes in csr_matrix.getcol/row
ENH: Add ext parameter to UnivariateSpline

Pass parameter to fitpack.splev to control how to handle out of range
evaluations. Parameter ext can be passed via __init__ or __call__. Added
test to scipy/interpolate/tests/test_fitpack2.py. Tests if UnivariateSpline
call equals x ** 3 for out of range x and extrapolation modes 0 and 1.

See ticket 3557.

closes gh-3557
BUG: use correct MAXGAM in beta.c

AFAICT the correct constant is used everywhere in cephes.

Also try harder to use the best order of operations when
calculating beta from gammas.
BUG: don't try to calculate beta in betainc, just call it.

fixes gh-3761
ENH: Add ext parameter to UnivariateSpline

Pass parameter to fitpack.splev to control how to handle out of range
evaluations. Parameter ext can be passed via __init__ or __call__. Added
test to scipy/interpolate/tests/test_fitpack2.py. Tests if UnivariateSpline
call equals x ** 3 for out of range x and extrapolation modes 0 and 1.
Modify _from_tck, derivative, and antiderivative methods to use ext
parameter. __init__ of LSQUnivariateSpline and InterpolatedUnivariateSpline
now accept ext.

See ticket 3557.

closes gh-3557
BUG: Fix lombscargle ZeroDivisionError

Change calculation of tau to use atan2 function instead of atan.

closes gh-3787
Merge pull request #3788 from Jacobcvt12/lombscargle-atan

BUG: Fix lombscargle ZeroDivisionError
TST: lombscargle regression test related to atan vs atan2
MAINT: put more classes back into the scipy.stats.distributions namespace
MAINT: attempt py3 compatibility fix
Merge pull request #3803 from argriffing/lombscargle-atan2-test

TST: lombscargle regression test related to atan vs atan2
MAINT: move redundant code into a function
MAINT: add code comment and make a small code change
BUG: numpy version checking in ndimage
Merge pull request #3806 from argriffing/ndimage-numpy-version

BUG: numpy version checking in ndimage
ENH: orthogonal procrustes solver
MAINT: replace asarray with more specific format request
MAINT: tweak import