docs: def center_genes(self, use_median=False, inplace=False):
          """Center the expression of each gene (row)."""
          if use_median:
              X = self.X - \
                  np.tile(np.median(self.X, axis=1), (self.n, 1)).T
          else:
              X = self.X - \
                  np.tile(np.mean(self.X, axis=1), (self.n, 1)).T
  
          if inplace:
              self.X[:,:] = X
              matrix = self
          else:
              matrix = ExpMatrix(genes=self.genes, samples=self.samples,
                                 X=X)
          return matrix