Consistent boundary enumeration for high-dimensional splines

[jzwar]

For some applications it would be useful to have a consistent enumeration of boundaries. The numbering system is somehow arbitrary and changes depending on the FE-System in use. As we are theoretically independent of the dimension, I would suggest the following numbering system

$$i_{face} = \left\{\begin{array} 2*j \quad\text{if face is at $\xi_j=0$}\\\ 2*j + 1 \quad\text{if face is at $\xi_j=1$} \end{array}\right.$$

for $j\in{0,\dots, n_{dim}-1}$ axis dimensions. This is applicable to higher dimensions as well

This would require to change the layout of the extract_boundary routine

[jzwar]

There is a displaying issue here. the first line should read 2*j

[jzwar]

Related to #77

[j042]

sure