Deprecate the Slurm functionality in this package (and point users to SlurmClusterManager.jl instead)

[DilumAluthge]

Ref #58.

[DilumAluthge]

Before we do this, it would probably be good to move SlurmClusterManager.jl to the JuliaParallel GitHub org. I've asked the package author if he's open to it.

[DilumAluthge]

Instead of deprecating, we could just list both options (ClusterManagers.SlurmManager and SlurmClusterManager.SlurmManager), and list the differences between the two. Then, users can decide which package they want to use.

We can probably borrow a lot of the language from the "Differences from ClusterManagers.jl" section here: https://github.com/kleinhenz/SlurmClusterManager.jl/blob/master/README.md#differences-from-clustermanagersjl

cc: @kleinhenz

[DilumAluthge]

Closing in favor of #246.