`sourmash_args.load_picklist` requires the entire namespace `args` instead of only the picklist object arguments

[ccbaumler]

The load_picklist method of sourmash_args requires the entire args namespace instead of filename:colname:coltype:picklistStyle. The need for an args object makes it difficult to code in simple scripts or nested functions. Namely, the ones used in sourmash plugins like https://github.com/dib-lab/sourmash_plugin_pangenomics/blob/main/src/sourmash_plugin_pangenomics.py

See L34 of a simple picklist script I wrote.

[ctb]

looking at src/sourmash/picklist.py, there's a class method that might suit:

sourmash/src/sourmash/picklist.py

Line 127 in 349b238

def from_picklist_args(cls, argstr):

which is what load_picklist uses anyway. I guess the reason not to use it is because of all the nice checking that the load_picklist method does.

I think the solution in #3839 breaks too much external code, too, since you now need to specify whether you're passing in an args or a picklist type.

We could, instead, add a load_picklist_from_str function that contains most of the code in load_picklist and then refactor load_picklist to use load_picklist_from_str internally. Then everyone using load_picklist keeps on using it as normal, and we can use the new function in new code. Thoughts?