[GeoMechanicsApplication] Add a class that represents a (p, q) vector

[avdg81]

As a developer, I would like to have a class that represents a ($p$, $q$) vector. This class will be very similar to the class that represents a ($\sigma$, $\tau$) vector.

Class design

• The class will store the values of $p$ and $q$ in a fixed-size array mValues (with a size of 2).
• The class will have member functions P and Q that return references to the corresponding elements of the array. This will make it easier to read expressions that operate on the component level.
• The class will be default-constructible, which will yield an instance with $p = 0$ and $q = 0$.
• The class can also be instantiated with any vector of size 2. In addition, a constructor will be provided that receives a std::initializer_list<double> (which must have a size of 2). Attempting to instantiate this class with a vector or initializer list with an incorrect size results in throwing an exception.
• The class will have a template member function CopyTo that copies the internal array to a container of the specified type. This will help to pass the values of $p$ and $q$ to functions that expect a vector type, e.g. Vector.

Needless to say, all the above mentioned functionality should be covered by unit tests.