Release v1.3.0

[tbekolay]

This is the release commit for v1.3.0, and a commit to fix / xfail some recent Nengo changes:

• Raise an error if sim is run for < dt seconds nengo#1357 changed what happens in sim.run; those changes were ported back here no problem
• Fixes bug where multiple identical indexed slices are ignored nengo#1361 fixed a bug with repeated indices in lists used to slice Nengo objects; those tests fail here so I xfailed them for now
• Piecewise param nengo#1355 introduced a Piecewise process with interpolation. The interpolation tests fail because we're evaluating the function at t=0.1, which in Nengo OpenCL's float32 in slightly more than 0.1, resulting in the interpolation function returning 0, which is the fill value. There are some things we could do the fix this (namely having the interp1d extrapolate instead of filling with 0) but that'll take some discussion, so xfailing it for now so we can get the release done

[hunse]

LGTM.

I added a commit to raise an error if using duplicate indices. This should be moved before the release commit.

[tbekolay]

Great, will do. Thanks!