Expose Dof Index

[matekelemen]

A requirement for solving #13160. This PR makes it possible to refer to Dofs with two integers (node ID, dof index within node) instead of a direct pointer.

[loumalouomega]

Expose into python too, LGTM

[RiccardoRossi]

Btw, I have no issues with this PR (other than the comment from @loumalouomega).

There might be a problem with the usage though: as i remember the list of dofs is dynamic, so that if you add or remove dof the index will change

[matekelemen]

There might be a problem with the usage though: as i remember the list of dofs is dynamic, so that if you add or remove dof the index will change

That's fine. What I need is the index of the Dof's variable within the VariablesList of the node and as far as I see, the order in there does not change during extension.

Kratos/kratos/containers/variables_list.h

Lines 282 to 300 in a14ec2f

	int AddDof(VariableData const* pThisDofVariable){
	for(std::size_t dof_index = 0 ; dof_index < mDofVariables.size() ; dof_index++){
	if(*mDofVariables[dof_index] == *pThisDofVariable){
	return static_cast<int>(dof_index);
	}
	}
	#ifdef KRATOS_DEBUG
	if(OpenMPUtils::IsInParallel() != 0)
	KRATOS_ERROR << "attempting to call AddDof for: " << pThisDofVariable << ". Unfortunately the Dof was not added before and the operations is not threadsafe (this function is being called within a parallel region)" << std::endl;
	#endif
	mDofVariables.push_back(pThisDofVariable);
	mDofReactions.push_back(nullptr);
	KRATOS_DEBUG_ERROR_IF(mDofVariables.size()>64) << "Adding too many dofs to the node. Each node only can store 64 Dofs." << std::endl;
	return mDofVariables.size() - 1;
	}

I'll extend the docstring to clarify what mIndex really is.

EDIT: actually, that reordering is a problem. Is there are reason why Node::mDofs is kept ordered? All accesses use a linear search instead of a binary one.