use isequal instead of == in safe_diff

[vyudu]

This would make interpolation on symbolic arrays work, e.g.

@variables x[1:101]
t = 0.:0.01:1
LinearInterpolation(x, t)

would work as expected (would throw errors before).

[vyudu]

@ChrisRackauckas this would make things nicer for CasADi symbolic objects because it doesn’t have built in interpolation, so being able to just use DataInterpolations machinery would be nice

[ChrisRackauckas]

Put a test on this?

[vyudu]

Marked out the ones that work and don't in the test, should I document this soemwhere?

[ChrisRackauckas]

Open an issue to follow up on those.