Code cleanup for relecov_core

[Shettland]

This PR addresses several changes to the relecov_core code:

• Renamed files in relecov_core which had redundant names samples_handling.py -> samples.py
• Imports in relecov_core and relecov_dashboard have been updated accordingly to these new filenames
• Changed local from imports to include the whole library name in the import, in order to detect where the imported object come from in the code.
• All the imported functions have been renamed accordingly
• Included some #FIXME and #TODO tags in the code with comments to review the code

[Daniel-VM]

It looks good. Just few things to accomplish:

• There are a few missing refactorings of imports that need to be addressed globally in the PR.
• Local imports should follow a consistent structure accross files by importing the entire file rather than specific components.
• Follow plotly conventions when importing this library.
• There are aliases used inconsistently in local imports. We need to decide whether to use them for local imports or not.

[saramonzon]

I see some renaming left, where p.e "handling" can be removed from the file name

[Daniel-VM]

LGTM