Parsing step's list arguments is broken

[mcara]

@jemorrison reported that parsing list arguments of steps does not work. Indeed, I verified that running strun with

--steps.resample.output_shape=500,800

which used to work before, no longer works:

ValueError: Config parameter 'output_shape': the value "500,800" is of the wrong type.

CC: @stscieisenhamer

After some detective work, I determined that #57 broke support for list arguments.

[jdavies-st]

Note #69 is the same issue and has a solution available.