general readonly tag

[jmilloy]

Node traits tagged as 'readonly' or as 'attr' can be specified using keyword arguments during Node instatiation, but cannot be later modified. It uses the read_only flag under the hood but is a slightly more flexible.

class MyDataSource(DataSource)
    source = tl.Unicode().tag(readonly=True)
    my_attr = tl.Int().tag(attr=True)

>>> node = MyDataSource(source='mysource', my_attr=8)
>>> node.source = 'other'
...
traitlets.traitlets.TraitError: The "source" trait is read-only.
>>> node.my_attr = 6
traitlets.traitlets.TraitError: The "my_attr" trait is read-only.

[jmilloy]

@mpu-creare Try this on for size. Replaces #313.

[coveralls]

Coverage increased (+0.6%) to 90.636% when pulling b343713 on feature/readonly_tag into f0beb50 on develop.

[mpu-creare]

Looks good to me. I made a suggestion for a slightly simpler implementation, but you might have had a reason for what you did.