def load_to_array(self, keys):
        """
        This loads the data contained in the catalogue into a numpy array. The
        method works only for float data

        :param keys:
            A list of keys to be uploaded into the array
        :type list:
        """
        # Preallocate the numpy array
        data = np.empty((len(self.data[keys[0]]), len(keys)))
        for i in range(0, len(self.data[keys[0]])):
            for j, key in enumerate(keys):
                data[i, j] = self.data[key][i]
        return data