Add some doc strings

[kcpevey]

• Add doc strings to new methods
• Add examples as doc test

Also while waiting for tests to run I added a few "close figs"

[kcpevey]

There were some other doc strings that I could have added but I was concerned about working on top of @MarcoGorelli ad @CommonClimate on changing code.

[CommonClimate]

Good thinking @kcpevey . Is this ready for review, then, or will you open another PR once we're done with code edits, so you can put in the other docstrings?

[CommonClimate]

Looks good! I might just change "pyleo_series" to "ts" in the load_dataset() docstring, as we typically use that moniker for a Series, the same way pandas uses "df" for dataframe, even though anything is allowed.

[CommonClimate]

Also, just saw the docstring for sel(), which is nice and comprehensive. In terms of the screen output, though, it won't look like this anymore:
{'log': ({0: 'clean_ts', 'applied': True, 'verbose': False}, {2: 'clean_ts', 'applied': True, 'verbose': False})}

It should be:

{'log': ({0: 'dropna', 'applied': True, 'verbose': True}, {1: 'sort_ts', 'direction': 'ascending'})}

Sorry to change this on you. So I guess this way of making docstrings does not take changes in the codebase into account. That is an argument for using ipython directives...

[kcpevey]

I might just change "pyleo_series" to "ts" in the load_dataset() docstring, as we typically use that moniker for a Series, the same way pandas uses "df" for dataframe, even though anything is allowed.

I can make that change to this PR.

Screen output for sel. That is the section I'm avoiding on working on so I will wait until the dust settles and open another PR.

[CommonClimate]

OK, I like that plan. Re: docstrings, the SeriesResampler will need a lot of explanation. Honestly, I still don't understand exactly how it works, and I doubt our users will. If applicable, you can link to this but also important to explain the differences. Or we may need a whole tutorial about how to use that class, preferably as a notebook in the paleopandas repo. We can then fold it into our PyleoTutorials.