refactor voltage initializer module

[EtienneLt]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem? no

What kind of change does this PR introduce? it refactors the code of voltage initializer.

What is the current behavior? all class are in the same file for this module

What is the new behavior (if this is a feature change)? it splits the code with one file per class

Does this PR introduce a breaking change or deprecate an API?

• [ ] Yes
• [x] No

What changes might users need to make in their application due to this PR? (migration steps) nothing