Add PeriodicCallback.

[tkoolen]

This is to support stuff like JuliaRobotics/RigidBodySim.jl#5.

I'm aware of http://docs.juliadiffeq.org/latest/features/diffeq_arrays.html#Example:-A-Control-Problem-1, but I wasn't satisfied with simply using the tstops kwarg to solve. Reasons:

1. I want support for infinite tspans (where integration continues indefinitely until some callback terminate!s the integrator, in my case a stop button in a GUI)
2. Even for finite tspans you'd end up with a length of the internal tstops collection that is linear in the length of the integration time interval with the other method; that's not the end of the world, but it seems unnecessary.
3. With PeriodicCallbacks, you only have to pass the callback into solve or init. I think this is cleaner than having to pass in both a callback and tstops, especially if there are multiple callbacks that each have a different period.

[ChrisRackauckas]

yeah, there must be a better way to iterate through a heap

[tkoolen]

Yeah, I guess it would at least be better to reverse-iterate.

Could also:

• store the maximum time in the integrator options, in addition to tstops (although it then becomes possible to have bugs where tstops is out of sync with that maximum time).
• switch to a min-max heap (not yet implemented in DataStructures.jl and perhaps overkill)

[coveralls]

Coverage increased (+0.5%) to 94.048% when pulling 35f6b4c on tkoolen:periodic-callback into e602c07 on JuliaDiffEq:master.

[codecov]

Codecov Report

Merging #23 into master will increase coverage by 0.6%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #23     +/-   ##
=========================================
+ Coverage   93.54%   94.15%   +0.6%     
=========================================
  Files           5        6      +1     
  Lines         155      171     +16     
=========================================
+ Hits          145      161     +16     
  Misses         10       10

Impacted Files	Coverage Δ
src/periodic.jl	100% <100%> (ø)

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[coveralls]

Coverage increased (+0.6%) to 94.152% when pulling 0552862 on tkoolen:periodic-callback into e602c07 on JuliaDiffEq:master.

[coveralls]

Coverage increased (+0.6%) to 94.186% when pulling b5c7a24 on tkoolen:periodic-callback into e602c07 on JuliaDiffEq:master.

[tkoolen]

Added README.md entry, made it so that passing in an initialize kwarg also works, and made things work for negative tdir (requires SciML/OrdinaryDiffEq.jl#210).

Are you satisfied with the tstops.valtree iteration changes? Anything else?

[ChrisRackauckas]

I wish there was a more generic way to iterate through a heap than that, but that's an issue for DataStructures.jl. Works for now.