Improve display of dimensionless quantities in tables

[astrofrog]

In the following example, it's not clear in the table repr that a is a dimensionless quantity as opposed to a unitless array for which units haven't been set:

In [1]: from astropy import units as u                                                                               

In [2]: from astropy.table import QTable                                                                             

In [3]: tab = QTable()                                                                                               

In [4]: tab['a'] = [1, 2, 3] * u.one                                                                                 

In [5]: tab                                                                                                          
Out[5]: 
<QTable length=3>
   a   
       
float64
-------
    1.0
    2.0
    3.0

(this happens for Table` too). Maybe we should consider including (dimensionless)`` in the unit row or something similar?

[Harshil-C]

@astrofrog i added dimensioless property in the unit row but there was a confusion when there is a dimensionless quantity with scale != 1.0 .In the code it speacifies an output
if self._scale != 1.0: return 'Unit(dimensionless with a scale of {})'.format(self._scale).
So i decided to keep that as it is because i thought it was an important message and putting it into the table header will generally make the plaintext render of the table useless. If you like to suggest any other way for that kindly let me know.
here in wrote a PR