Add build and html directories

This PR does some small optimisations to setLcs to avoid map / set lookups. For my test case the comparison the inner loop is not called very often so the length computation is delayed as long as possible.

Some minor changes to the numerics for better code generation and I added a hypotenuse function to allow code-reuse.

In addition the build directory and the html doxygen output have been added to the gitignore.