[1/3]second fails as valid duration input

[viboes]

[1/3]second

makes the code

seconds d; cin >> d

fail at run-time because the singular form is not associated to the keywords for the form [N/D].

The correction is easy. Just to add a line to the scanned units the singular form in addition to the plural and the symbol.

[viboes]

Fixed in trunk r75237.

[viboes]

(In [75665]) Chrono: Fix #6092,#6093,#6113