Move registration of ConnBuilders, Backends, growth curves from nestmodule to respective managers, allowing unloading

[heplesser]

This PR is a follow-up to #3103 and should be merged only after it (the set of changes in this one then becomes much smaller). It moves the registration of ConnBuilders, Recording/StimulationBackends and SP growth curves from nestmodule::init() to the initialize() methods of Connection, IO and SP Manger respectively, with corresponding clean-up in the finalize() methods. The actual changes are small, since nestmodule::init() in the past called register_* methods in the respective managers.

Thus, these elements will be unloaded on ResetKernel() and such elements provided by extension modules can be reloaded. Therefore, if at all possible, this should still make it into 3.7 together with #3103.

Parameters and Masks should be re-organizedi in the same way, but they are fully managed by nestmodule and seemingly with a rather complicated approach (parameters are also available as SLI objects), so their transition to the new scheme is more work and needs to wait.

[heplesser]

@pnbabu @terhorstd #3149 is now ready for review as #3103 has been merged. Two comments:

• In io_manager.cpp I have moved initialize() and finalize() forward in the file so that they appear right after constructor and destructor, as in the code for other managers.
• In KernelManager, io_manager must be initialized before model_manager as registration of stimulation and recording device models requires that the backends are registered.

[pnbabu]

Looks good to me.

[terhorstd]

LGTM 👍