public elemental function tem_FirstIdAtLevel(level) result(res)

This function delivers the refinement level of a TreeID

The refinement level of a
\ref treelmesh_module::treelmesh_type::treeid "treeID" is found by
repeatedly computing the parents with the equation used in the
[tem_ParentOf function] (@ref tem_parentatlevel) and
counting the iterations, until the universal root is reached.

First TreeID in the complete tree on a given level

A level offset is calculated by summing all the nodes of coarser levels.
With a level count starting at \f$0\f$ on the coarsest level (the universal
root node), this offset \f$s\f$ for a given level \f$L\f$ can be computed by
the follwing equation:
\f[
s = \sum\limits_{i=0}^{L-1} 8^i = \frac{8^{L} - 1}{7}
\f]

Todo

: calling to this function can be replaced by accessing array
firstIdAtLevel which explicitly stores the results

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