Returns a quantity specification for docstrings.

Example
-------

>>> quantity_spec_string('Tv')
>>> 'Tv : float or ndarray\n    Data for virtual temperature.'

def quantity_spec_string(name, quantity_dict):
    '''
Returns a quantity specification for docstrings.

Example
-------

>>> quantity_spec_string('Tv')
>>> 'Tv : float or ndarray\n    Data for virtual temperature.'
    '''
    if name not in quantity_dict.keys():
        raise ValueError('{0} not present in quantity_dict'.format(name))
    s = '{0} : float or ndarray\n'.format(name)
    s += doc_paragraph('Data for {0}.'.format(
        quantity_string(name, quantity_dict)), indent=4)
    return s