Fix DataFrames compatibility

[gabrieldansereau]

What the pull request does

DataFrames tests are currently failing because of a change between DataFrames v0.22.x and v1.0.0. I'll fix it and take a look at our compat entries at the same time.

Edit: Problem is fixed and the tests now pass. SimpleSDMLayers now works with DataFrames versions past v1.0. It still works with the v0.22.x series, but not v0.21.x. I removed the compat entry in the tests for v0.21.x.

Type of change

Please indicate the relevant option(s)

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ❇️ New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• 📖 This change requires a documentation update

Checklist

• The changes are documented
  • The docstrings of the different functions describe the arguments, purpose, and behavior
  • There are examples in the documentation website
• The changes are tested
• The changes do not modify the behavior of the previously existing functions
  • If they do, please explain why and how in the introduction paragraph
• For new contributors - my name and information are added to .zenodo.json
• The Project.toml field version has been updated
  • Change the last number for a v0.0.x series release, a bug fix, or a performance improvement
  • Change the middle number for additional features that do not break the current test suite (unless you fix a bug in the test suite)
  • Change the first number for changes that break the current test suite

[codecov-commenter]

Codecov Report

Merging #96 (a0eec86) into master (993f286) will decrease coverage by 7.56%.
The diff coverage is 68.93%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   88.01%   80.45%   -7.57%     
==========================================
  Files          22       24       +2     
  Lines         609      752     +143     
==========================================
+ Hits          536      605      +69     
- Misses         73      147      +74     

Flag	Coverage Δ
unittests	80.45% <68.93%> (-7.57%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/recipes/recipes.jl	90.47% <ø> (-2.63%)	⬇️
src/datasets/types.jl	26.66% <26.66%> (ø)
src/datasets/geotiff.jl	64.13% <45.00%> (-32.30%)	⬇️
src/datasets/earthenv/download.jl	84.37% <84.37%> (ø)
src/SimpleSDMLayers.jl	92.30% <100.00%> (+4.80%)	⬆️
src/datasets/ascii.jl	92.68% <100.00%> (+0.37%)	⬆️
src/datasets/chelsa/bioclim.jl	100.00% <100.00%> (ø)
src/datasets/chelsa/download.jl	100.00% <100.00%> (ø)
src/datasets/earthenv/habitatheterogeneity.jl	100.00% <100.00%> (ø)
src/datasets/earthenv/landcover.jl	100.00% <100.00%> (ø)
... and 9 more

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