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
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
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
TST: add tests for ext parameter of {Interpolated,LSQ}UnivariateSpline

Light smoke tests only.
ENH: interpolate: string keywords to control extrapolation in fitpack2

splev accepts ext=0, 1, 2; class-based wrappers take `ext` as either
int, or a string value.
Merge pull request #2 from ev-br/pr-3773_

Pr 3773
Merge pull request #3773 from Jacobcvt12/UnivariateSpline

ENH: Add ext parameter to UnivariateSpline call