vcs should rember the original value stored by the user

[chaosphere2112]

example

>>> gm  = x.createisoline()
>>> gm.line= ['dot','dash']
>>> gm.line
['dot', 'dash']
>>> gm.line= [1,2,3]
>>> gm.line
['dash', 'dot', 'dash-dot']
>>> 

Migrated from: https://github.com/UV-CDAT/uvcdat/issues/2010