implement context management with the `with` keyword

[zshaheen]

I think cdms should support context management with the with keyword. So the idea is to have something similar to how files are handled in Python:

with cdms2.open('precc.nc') as precc_file:
   var = precc_file('PRECC')

This automatically has exception handling and automatically closes the file after leaving the scope. All that is needed is to implement __enter__() and __exit__() functions.

[doutriaux1]

should be fixed by #121