Returns the value of a dimension.

        Args:
            dimname: Name of the variable
            path: path to the group.
            default: return `default` if `dimname` is not present and
                `default` is not `NO_DEFAULT` else raise self.Error.

def read_dimvalue(self, dimname, path="/", default=NO_DEFAULT):
        """
        Returns the value of a dimension.

        Args:
            dimname: Name of the variable
            path: path to the group.
            default: return `default` if `dimname` is not present and
                `default` is not `NO_DEFAULT` else raise self.Error.
        """
        try:
            dim = self._read_dimensions(dimname, path=path)[0]
            return len(dim)
        except self.Error:
            if default is NO_DEFAULT: raise
            return default