Added manual threshold algorithm

[volks73]

Simple implementation to apply thresholding with manual lower and upper limits. The auto-thresholding algorithms can be used to automatically select a threshold and apply it, but a function to simply apply user determined limits could be useful.

[volks73]

I forgot to add a unit test for the "manual" threshold. However, in going back and implementing the unit test, I realized the "manual" algorithm is just an application of a calculated or determined threshold value. So, I changed the ThresholdManualExt trait to ThresholdApplyExt and simplified a little bit by combining the range feature with the existing apply_threshold private function.

I also refactored the imports for the development to be within the tests sub-module.

Something to consider is renaming the threshold_otsu and threshold_mean functions for the ThresholdOtsuExt and ThresholdMeanExt traits, respectively. I would suggest breaking them into two separate functions, one that calculates the threshold level and the other that calculates and applies, for example:

...
fn threshold_calculate_otsu(&self) -> Result<f64, Error>;

fn threshold_apply_otsu(&self) -> Result<Self::Output, Error>;
...

The threshold_apply_otsu would be the current implementation, just a renaming to be clear it is applying the threshold as calculated by the Otsu method, while the threshold_calculate_otsu returns the calculated threshold value that could be used with the threshold_apply function or for further processing/algorithms.

[xd009642]

Sure I'm fine with splitting threshold calculation and application 👍. PR overall looks good so far, if you also update the changelog adding a new [Unreleased] field at top and noting down any changes (I won't review that too stringently it's more so next release I don't forget anything done when finalising changelog)

[volks73]

I have added the [unreleased] section to the CHANGELOG.md file.

Separating out the apply from the calculate as different methods should be addressed in a separate PR since it could potentially break compatibility with previous versions.

[xd009642]

Sure, if you want to make an issue to track it or just go straight into the PR if you want to I'll just give this a last looksie and then merge if it still looks all good