added gray image operation

[chamodperera]

Description

Added functionality to gray an image. Now the "gray the image" block under the convertions section can be used to gray an image.

• Feature Operations in the convertions section are not working #135

Type of change

• 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 not to work as expected)
• This change requires a documentation update

How Has This Been Tested?

• Tested manually

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

[SahanDisa]

Can you please check the merge conflicts and open the PR again @chamodperera ?