Compatibility with CASA 5.0

[jmoldon]

For CASA 4.* we need the lines:

from casa import table as tb
from casa import ms

Without them, it cannot find the tb tasks, for example.

However CASA 5.0 crashes if those lines are there, but works fine if not.

We may implement a try/except chek of the CASA version, but I think it is better if we can find a common way to load all the CASA tasks uniformly, if possible. Also, we should get rid of:

from tasks import *
from casa import *

We should check any changes with that at least in CASA 4.7 and CASA 5.0.

[jmoldon]

v0.6 compatible only with CASA 5, no previous versions