feat(sim): Allow branch registration in `FairGenerator`

[karabowi]

Introduced new function FairMCApplication::InitFinalizer() which initializes event generator, tasks, and triggers FairRootManager::WriteFolder() function. This new function is called from the latter of InitGeometry() and AddIons().

Previously the funcionality of InitFinalizer was split between InitGeometry() and AddIons(), which caused problems because TGeant3 calls AddIons() first, InitGeometry() second, TGeant4 calls InitGeometry() first, AddIons() second.

Current implementation assures that initialization of event generators and tasks come before WriteFolder.

Fixes issues #1183 and #1567.

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Checklist:

• [x] Followed the Contributing Guidelines