Return the trixel corresponding to the given htmid (htmid is the unique integer identifying each trixel).
Note: this method is not efficient for finding many trixels. It recursively generates trixels and their children until it finds the right htmid without remembering which trixels it has already generated. To generate many trixels, use the get_all_trixels() method, which efficiently generates all of the trixels up to a given mesh level.
Note: valid htmids have 4+2n bits with a leading bit of 1