check if @pretty_decorator makes the dependency on multiprocess obsolete

[Thomas Gilgenast]

currently lib5c depends on multiprocess to allow pickling of functions parallelized via @parallelize_regions

since c095acb @parallelize_regions is itself decorated with @pretty_decorator. it is unknown whether or not the new mechanics of decorators decorated with @pretty_decorator allow them to be pickled without dill, but we should investigate this

[Thomas Gilgenast]

we dug into this and made the following observations:

• when decorator is not available, @parallelize_regions breaks on some rare edge cases related to default kwargs
• even when decorator is available, functions decorated with @pretty_decorator decorators still cannot be pickled
• it is possible to drop the multiprocess dependency if you retain dependence on dill and use that package to dodge the pickling problem

the recommendations are:

• make decorator a required dependency
• remove multiprocess from the dependencies, replacing it with dill

[Thomas Gilgenast]

refactored dependencies related to parallelization

decorator is now a required dependency

dill replaces multiprocess as a dependency

modified @parallelize_regions to use dill directly instead of multiprocess

closes #13

→ <<cset b2835c86f2ee>>