Create NEST Extension Interface for NEST compatible with new registration scheme and unloading

[heplesser]

The title says it all. This is still in draft stage as I need to iron out some problems, especially segfaults on lt_dlclose() of the module. ResetKernel() works and one can install the module again afterwards, but I have not yet checked what happens if the module changes between reloads. At present, I need to provide the (DY)LD_LIBRARY_PATH to the install location of the module. The old DynamicLoader is replaced by ModuleManager and NESTExtensionInterface is the base class for extension modules.

In nest/nest-extension-module#26 you will find a version of mymodule (in the modulemgr branch) that works with this branch of NEST.

This fixes #3064.

[heplesser]

Everything works now, including loading a modified module after ResetKernel(). One still needs to make sure that the path to the extension module is explicitly set in (DY)LD_LIBRARY_PATH, even if it is the same as the installation location for the NEST libs. Got to see if one can improve on this, would need some search-path expansion in the module manager. But otherwise, this is ready for review.

[pnbabu]

Thank you for these changes! I tested this with modules generated from NESTML and the install works fine. I have a few comments. Please find them below.

[heplesser]

@pnbabu Thank you for your review! I have fixed most items and commented on the two remaining.

[terhorstd]

Some minor points and some questions still open.

[pnbabu]

Looks good to me! Just some minor suggestions.

[heplesser]

@terhorstd Thanks for your comments! Could you take a look at my replies?

[terhorstd]

Looks good so far. Thanks for the complex work!