RgbToOd filter return np array with correct range and use correct formula

[alessiamarcolini]

Description

RgbToOd filter return np array with correct range and use correct formula

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Documentation

Issues Fixed or Closed by This PR

• Fixes:

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[codecov]

Codecov Report

Merging #331 (a4a4129) into master (437aa84) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #331   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           17        17           
  Lines         1310      1308    -2     
  Branches       128       128           
=========================================
- Hits          1310      1308    -2     

Impacted Files	Coverage Δ
histolab/filters/image_filters.py	100.00% <100.00%> (ø)
histolab/filters/image_filters_functional.py	100.00% <100.00%> (ø)

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[ernestoarbitrio]

2 small changes than LGTM