Allow multiple tasks to be set up with `-t` flag

[xylar]

This merge changes the -t or --tasks flag to polaris setup to allow multiple task to be specified.

Since there is not a sensible way to specify cached steps for each of several tasks, the --cached flag only works when a single task is specified to to -t or -n.

Checklist

• [x] User's Guide has been updated
• [x] Developer's Guide has been updated
• [x] Documentation has been built locally and changes look as expected
• [x] Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I was able to set up and run the single-column ocean tests on my laptop:

$ polaris setup -t ocean/single_column/cvmix ocean/single_column/ideal_age -p ...

Then,

$ source load_polaris_env.sh 
$ plaris serial
ocean/single_column/cvmix
  * step: init
          execution:        SUCCESS
          runtime:          0:00:01
  * step: forward
          execution:        SUCCESS
          runtime:          0:00:02
  * step: viz
          execution:        SUCCESS
          runtime:          0:00:01
  task execution:   SUCCESS
  task runtime:     0:00:04
ocean/single_column/ideal_age
  * step: init
          execution:        SUCCESS
          runtime:          0:00:01
  * step: forward
          execution:        SUCCESS
          runtime:          0:00:02
  * step: viz
          execution:        SUCCESS
          runtime:          0:00:01
  task execution:   SUCCESS
  task runtime:     0:00:04
Task Runtimes:
0:00:04 PASS ocean/single_column/cvmix
0:00:04 PASS ocean/single_column/ideal_age
Total runtime: 0:00:08
PASS: All passed successfully!

[xylar]

@sbrus89, this is ready to review.

[xylar]

@sbrus89, since this was a feature request, I'm going to leave it for you to review, unlike the recent bug fixes.