add `change_length` coord method

Description

This PR adds a method to CoordRange called change_length to allow easy modifications to the length of evenly sampled coordinates. It was primarily added to mitigate off by one errors related to float coordinates, but may be useful for other things to.

It looks like this:

new_coord = coord.change_length(length=10)  # change length to 10 by adjusting end values.

Checklist

I have (if applicable):

[ ] referenced the GitHub issue this PR closes.
[ ] documented the new feature with docstrings or appropriate doc page.
[ ] 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 Report

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

Project coverage is 99.55%. Comparing base (4330787) to head (3fe3e78).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #363 +/- ## ========================================== + Coverage 98.85% 99.55% +0.70% ========================================== Files 94 94 Lines 7392 7400 +8 ========================================== + Hits 7307 7367 +60 + Misses 85 33 -52 ``` | [Flag](https://app.codecov.io/gh/DASDAE/dascore/pull/363/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/363/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DASDAE) | `99.55% <100.00%> (+0.70%)` | :arrow_up: | 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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DASDAE / dascore