#7: add `coalesce`

[pearsonca]

Description

This PR closes #7.

Introduces a coalesce argument to coerceDT, which acts as select-if-available-otherwise-use-default

Checklist

• [X] My PR is based on a package issue and I have explicitly linked it.
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• [ ] I have added or updated unit tests where necessary.
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[pearsonca]

righto @seabbs - default working so far as i've tested. open to suggestions on other things to test