Delete AppDataBean ComputationManager fields to use those in AppData

Please check if the PR fulfills these requirements

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

What is the current behavior? The only way to reinitialize the connection manager is to call clean() and init() but these methods also reinitialize other backend connections (Cassandra ...)

What is the new behavior (if this is a feature change)? The existing clean() and init() methods are still there. A new method reinitComputationManager() is added to reinitialize the computation manager alone without impacting other backend

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

[ ] Yes
[x] No

If yes, please check if the following requirements are fulfilled

[ ] The Breaking Change or Deprecated label has been added
[ ] The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information:

powsybl / powsybl-afs

Delete AppDataBean ComputationManager fields to use those in AppData #152

Quality Gate passed