Expose ncclimo --parallel command line option

[golaz]

In some cases, I found it necessary to change the --parallel option in ncclimo to successfully generate time series or climo files with zppy. But zppy currently does not expose that option to the user and so one is forced to manually edit the bash script.

[forsyth2]

@golaz ncclimo is called in three places -- twice in climo.bash and once in ts.bash. The 2 climo calls had --parallel=mpi, so in #452 I changed the parameter value to be user-defined. The ts call doesn't use parallel though. Should it be conditionally called depending if the user has set a value for it?

[forsyth2]

Options:

• Define parallel = string(default="") in [default], parallel = string(default="mpi") in [climo]. As mentioned in Improve cfg parameter inheritance hierarchy #212, this means that the [climo] default would override a user-provided value in [default]. By default, climo would run with mpi and ts would run without the parallel option at all.
• Define parallel = string(default="mpi") in [default]. Have ts run with mpi by default, just like climo.

[forsyth2]

In #452, I go with the first of the above two options, to maintain current functionality by default.