* merged 'kmer32.h/cpp' and 'kmerXX.h/cpp' to new class 'kmer.h/cpp'
 * merged 'color64.h/cpp' and 'colorXX.h/cpp' to new class 'color.h/cpp'
 * moved filter/Newick code from 'graph.h/cpp' to new class 'tree.h/cpp'
 * improved performance, especially for small k-mers and colors (32-bit)
 * improved the readability of code in all the main classes in general