[Core] Exploit Unused Bits in Dof

[matekelemen]

This PR addresses #11916 and adds a restricted set of flags to Dof, but unlike #12196, it does not increase its size. The price to pay for it is fewer flags and more complex code.

Kratos/kratos/includes/dof.h

Lines 439 to 452 in 7bf35ed

	/** True is is fixed */
	int mIsFixed : 1;
	int mVariableType : 4;
	int mReactionType : 4;
	int mIndex : 6;
	/** Equation identificator of the degree of freedom */
	EquationIdType mEquationId : 48;
	/** A pointer to nodal data stored in node which is corresponded to this dof */
	NodalData* mpNodalData;

In total, 4 extra bits can be used in Dof without increasing its size:

• the bit size of Dof was 1 + 4 + 4 + 6 + 48 + 64 = 127, which means that one extra bit mIsActive could be squeezed into the list of member variables.
• NodalData has an alignment requirement of 8 bytes. This means that any valid pointer to a NodalData instance must be a multiple of 8 (2³). As a result, the lowest 8 bits of any NodalData* is always 000 and we can exploit this fact to hide 3 bits of some other information (such as 3 flags) there.

[loumalouomega]

Looks great, but compilation is failing

[loumalouomega]

We usually use
/**
*
*/
style

[matekelemen]

ready for review

[matekelemen]

Ping reviewers.

I understand if this, or the other PR, is not exactly the solution you want to see but you need to tell me that if it's the case. #11916 prohibits me from progressing with my work, and I need to know at least the interface of its solution (we can iron out implementation details later, but I need to know how I can get extra information from Dofs).