Add model_out_tbl subsetting functions

[annakrystalli]

This PR resolves #149 by

• Adding subset_task_id_names() function to subset task ID names from a character vector of column names.
• Adding functions subset_task_id_cols() and subset_std_cols() to subset a model_out_tbl or submission tbl to task ID or standard (non-task ID) columns respectively.

Note I used a different function name to that proposed in #149 as I feel it's more descriptive and also to avoid any confusion with functions like get_task_id_names() which fetches task ID names from a config file. In general most get_*() functions act on config files so I'd prefer to stick with that convention if that's ok.

I've also made it so that we can also use them on submitted tbl objects which lack the model_id column as they can also be useful in hubValidations

Also, note that the check to ensure that the input contains all task ID columns is not implemented in this function. That would require checking against the config which would require more inputs. To keep these functions lightweight, I leave that check to be performed in higher level functions that call the subset_*_cols() functions.

[github-actions]

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[elray1]

this is perfect, thanks! I know it's almost trivial functionality but it will be nice to clean this detail up in a few places.