fix badge links, doc build, python versions

[d-chambers]

Description

This PR adds a DASCore version badge, synchronizes the badges between the readme and landing page for the docs, and fixes the sentek module docstring.

In preparation for 0.1.0, I have also removed the "DASCore is very new" warning.

Checklist

I have (if applicable):

• [x] referenced the GitHub issue this PR closes.
• [x] documented the new feature with docstrings or appropriate doc page.
• [x] included a test. See testing guidelines.
• [ ] your name has been added to the contributors page (docs/contributors.md).
• [ ] added the "ready_for_review" tag once the PR is ready to be reviewed.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (6d38342) 99.50% compared to head (62bf36c) 99.50%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #323 +/- ## ======================================= Coverage 99.50% 99.50% ======================================= Files 92 92 Lines 7130 7130 ======================================= Hits 7095 7095 Misses 35 35 ``` | [Flag](https://app.codecov.io/gh/DASDAE/dascore/pull/323/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/DASDAE/dascore/pull/323/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE) | `99.50% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE#carryforward-flags-in-the-pull-request-comment) to find out more.

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