refactor(layers) Some fine-tuning of layers refactoring, cleanup, comments

[Alex-NRCan]

Description

• Moved updateGeojsonSource outside of map-event-processor to abstract the logic within the gvgeojson class itself
• Adding layers management inside the GVGroupLayer class
• Added a refresh and onRefresh functions in the AbstractBaseLayer for the all the GVLayers and GVGroupLayer classes.
• Further cleanup of getAttributions, getExtentFromFeatures

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

How Has This Been Tested?

Hosted June 19 @ 12:15 : https://alex-nrcan.github.io/geoview

Checklist:

• I have build (rush build) and deploy (rush host) my PR
• I have connected the issues(s) to this PR
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have created new issue(s) related to the outcome of this PR is needed
• I have made corresponding changes to the documentation
• 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

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This change is 

[jolevesq]

Reviewed 27 of 27 files at r1, 3 of 4 files at r2.
Reviewable status: 29 of 30 files reviewed, all discussions resolved (waiting on @Alex-NRCan)

[jolevesq]

Reviewed 1 of 4 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Alex-NRCan)

[jolevesq]

Reviewed 2 of 2 files at r3, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Alex-NRCan)