Synthesis of a water soluble boron neutron capture therapy agent: 1-amino-3-[2-(7-{3-[2-(2-hydroxymethyl-ethoxy)-1-(2-hydroxy-1-hydroxymethyl-ethoxymethyl)ethoxy]propyl}-1,7-di-carba-closo-dodecaboran-1-yl)ethyl]cyclobutanecarboxylic acid