This functions loads a BWT file and constructs total counts, indexes start positions, and constructs an FM index in memory
        @param dirName - the directory to load, inside should be '<DIR>/comp_msbwt.npy' or it will fail

def loadMsbwt(self, dirName, logger):
        '''
        This functions loads a BWT file and constructs total counts, indexes start positions, and constructs an FM index in memory
        @param dirName - the directory to load, inside should be '<DIR>/comp_msbwt.npy' or it will fail
        '''
        #open the file with our BWT in it
        self.dirName = dirName
        self.bwt = np.load(self.dirName+'/comp_msbwt.npy', 'r')
        
        #build auxiliary structures
        self.constructTotalCounts(logger)
        self.constructIndexing()
        self.constructFMIndex(logger)