add more parallel options

Suggestion On Iago's SSMSE JOSS review:

Packages 'foreach' and 'doParallel' are only 'Suggests' in DESCRIPTION, but foreach() is used in package code, although its presence is checked. If option 'parallel=TRUE' is chosen, the code sets up a parallel cluster using parallel::makeCluster, when other backends could be employed, e.g. doFuture. An alternative option would be to import the 'foreach' package, and leave the setup to the user. For example, a Linux user would choose a 'FORK' cluster rather than the default 'PSOCK', for efficiency reasons.

@nathanvaughan-NOAA is working on some other parallel code for another project, so we can build upon his work after it is done.