Gracefully handle missing optional dependencies

[LukeWeidenwalker]

This issue is to ensure that missing optional imports only cause those processes to fail that actually require them.

From the discussion in #177:

There are several approaches to handling optional dependencies:

• local "just in time" imports (instead of global module level imports)
• falling back on None, e.g.

try:
    import dask
except ImportError:
    dask = None

Duplicate of #5.