Various minor improvements

[gabrieldansereau]

What the pull request does

A few minor improvements before the next release.

• [x] Make Requires loading calls similar to the ones in GBIF.jl
• [ ] ~Add a geotiff(SimpleSDMPredictor, file; coords) similar to layer[coords] (as combining layer[coords] with geotiff(SimpleSDMPredictor, file; coords...) feels a little inconsistent)~
  • Requires to define way too many additional methods, would be confusing and a pain to maintain
• [x] Add == and isequal overloads
• [x] Add an hash overload
• [x] Add more informative error message for replace!(SimpleSDMPredictor, ...)
• [x] Fix the layer1[layer2] method (closes #107)
• [x] Add a boundingbox(layer) = (left = layer.left, right = layer.right, bottom = layer.bottom, top = layer.top) function
• [x] Fix geotiff when using a custom nodata value (closes #108)

Type of change

Please indicate the relevant option(s)

• [x] :bug: Bug fix (non-breaking change which fixes an issue)
• [x] :sparkle: New feature (non-breaking change which adds functionality)
• [x] :boom: Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] :book: This change requires a documentation update

Checklist

• [x] The changes are documented
  • [x] The docstrings of the different functions describe the arguments, purpose, and behavior
  • [x] There are examples in the documentation website
• [x] The changes are tested
• [x] The changes do not modify the behavior of the previously existing functions
  • If they do, please explain why and how in the introduction paragraph
• [x] For new contributors - my name and information are added to .zenodo.json
• [x] 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 #106 (d76e640) into master (e6f3edc) will increase coverage by 0.63%. The diff coverage is 81.81%.

@@            Coverage Diff             @@
##           master     #106      +/-   ##
==========================================
+ Coverage   81.74%   82.38%   +0.63%     
==========================================
  Files          24       24              
  Lines         778      789      +11     
==========================================
+ Hits          636      650      +14     
+ Misses        142      139       -3     

Flag	Coverage Δ
unittests	82.38% <81.81%> (+0.63%)	:arrow_up:

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

Impacted Files	Coverage Δ
src/SimpleSDMLayers.jl	80.00% <ø> (ø)
src/datasets/geotiff.jl	73.68% <60.00%> (ø)
src/lib/basics.jl	100.00% <100.00%> (ø)
src/lib/overloads.jl	91.25% <100.00%> (+2.58%)	:arrow_up:

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