Consider using find_or_emplace

[domfournier]

discretize/discretize/_extensions/tree.cpp

Line 172 in 90f8482

key = key_func(x, y, z);

From K Davis.
There should be a faster method to directly add faces without touching the vector 3 times.

[jcapriot]

The method is try_emplace that conditionally inserts elements into maps in C++, and is available in the C++17 standard, and it would easily improve the mesh creation time, as currently every time a new item (node, face, edge) is added to the map, it requires 2 searches (not 3).

This does mean we would be forced to use compilers which support this standard, which all of the recent ones do now so that shouldn't be too much of an issue. The only minor headache is that the compiler argument passed to msvcc is \std:c++17 and the argument passed to gnu/clang compilers is -std=c++17.

[jcapriot]

Added in #308