Change Contours size to be `f64` instead of `usize` #327

Job	Run time
Code Coverage	2m 0s
Clippy	2m 55s
Build Examples (3d_charts)	34s
Build Wasm Examples (wasm-yew-minimal)	47s
Build Examples (basic_charts)	25s
Rustfmt	13s
semver	1m 11s
Build Examples (custom_controls)	24s
Tests (ubuntu-latest)	2m 6s
Build Examples (financial_charts)	24s
Tests (windows-latest)	3m 45s
Build Examples (images)	38s
Tests (macos-latest)	2m 28s
Build Examples (kaleido)	1m 10s
Build Examples (maps)	28s
Build Examples (ndarray)	27s
Build Examples (scientific_charts)	27s
Build Examples (shapes)	24s
Build Examples (subplots)	23s
	21m 9s

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