Reduce compile time by allowing explicit template instantiations

[philippgs]

We need to investigate how to allow explicit template instantiations of API constructs in user applications in order to reduce the compile time. Specifically, we need to check whether such explicit template instantiations are compatible with in-class member function definitions to keep the code clean and readable.

[HerbertJordan]

For explicit instantiations of member functions definitions need to be provided outside the class definition, separated from its declaration.

However, explicit external template instantiations need to be declared within the same name as the template class itself. This makes it a less practical approach for reducing the compile time of user code. User code would have to extend User API constructs to get control over template instantiations.

We are investigating alternative means for reducing the compile time. The main idea is to reduce the amount of classes and functions that are required to be instantiated when working with meshes.