feat: only download FES files if non-existent or updated

[codecov[bot]]

Codecov Report

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

Project coverage is 69.42%. Comparing base (76a288a) to head (38b045e). Report is 3 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #308 +/- ## ========================================== + Coverage 69.34% 69.42% +0.07% ========================================== Files 39 39 Lines 8844 8942 +98 Branches 1195 1208 +13 ========================================== + Hits 6133 6208 +75 - Misses 2325 2348 +23 Partials 386 386 ``` | [Flag](https://app.codecov.io/gh/tsutterley/pyTMD/pull/308/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tyler+Sutterley) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/tsutterley/pyTMD/pull/308/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tyler+Sutterley) | `69.42% <100.00%> (+0.07%)` | :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=Tyler+Sutterley#carryforward-flags-in-the-pull-request-comment) to find out more.

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