Add support for shared steps

[xylar]

This merge adds support to the polaris framework for shared steps (steps that can belong to more than one task). Existing tasks and steps are modified to accommodate the changes to the framework but tasks and steps are not yet reorganized to take advantage of shared steps so that bit-for-bit comparison with main can still be made with this branch.

Changes to Step include:

• switch the subdir attribute to be relative to the component work directory, not the task work directory
• add an indir parameter to the constructor to easily construct a subdir that joins indir and name. If indir is the task work directory, this is the same as the old default behavior when subdir was not specified.

Changes to Component include:

• adding a dictionary of steps (with work-directory paths relative to the component's work directory as keys)
• add add_step() and remove_step() methods

Changes to Task include:

• modify the add_step() method to perform error checking (e.g. make sure a different step with the same path hasn't already been added) and then add the step to the component and the task. This method also includes a new symlink parameter that can be used to make a local symlink in the task's work directory to a shared step if it resides outside the task.
• add remove_step() methods

Other changes include:

• listing steps with full work-directory paths when polaris list --verbose is called
• removing tasks from step pickle files (since shared steps will not belong to a single task)
• add a polaris_step_complete.log file once a step has run to indicate that it is complete (and should not be run again)
• add --clean flags to polaris setup and polaris suite to remove the base work directory before setting up tasks. (This is sometimes useful when debugging because steps that have already run will now be skipped, rather than running again, which may not be what the developer wants.)
• adding a resolution_to_subdir() function to the ocean framework for conveniently converting a float resolution to a string.

Checklist and changes look as expected

• [x] Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I have run all test cases except 1 and 4 km RPE tests and made sure results are BFB with main.

[xylar]

I don't plan on updating the documentation here. Instead, I will do that in #117

[xylar]

@cbegeman, thanks very much for the review. These are all great suggestions, which I will make tomorrow.

[xylar]

@cbegeman, I believe I've addressed your 3 requested changes. Please have another look when you have time.

[xylar]

@cbegeman, thanks for looking it over again. I think we're good for now. Let's see what @sbrus89 says.

[xylar]

@sbrus89, thanks so much for your comments! I appreciate you taking a look. Let me know if I've addressed your comments or if you'd like to see more changes.

[xylar]

Thanks so much!