TraitType -> BaseDescriptor, and replace call to set_default_value with instance_init

[SylvainCorlay]

• Missed a replacement of TraitType by BaseDescriptor.
• add_trait should call instance_init, not set_default_value. Then there is no remaining call to set_default_value, which I remove.

[SylvainCorlay]

Ping @minrk, @takluyver ? this one is quite trivial.

[minrk]

It's surprising to me that simply removing the call to set_default_value has has no effect. instance_init doesn't appear to set the default value, so functionality does seem to be removed. Can you explain that a little bit?

[SylvainCorlay]

• instance_init should clearly be called in add_trait. (e.g. for setting name and this_class of sub-traits)
• The last call to the set_default_value method was in add_trait. It is actually not needed.

Other calls were removed by earlier PRs. A call was removed when we decided to wait to see if a value was provided in a keyword argument of the constructor of the HasTraits instance. Another call was removed when Thomas introduced the _set_default_value_at_instance_init method.

• Example after this PR:

from traitlets import *
class Foo(HasTraits):
    pass
foo = Foo()
foo.add_trait('bar', Int(1)) 
print(foo.bar )  # prints 1 as expected because self.init_default_value is called in `__get__`.

[minrk]

ok. That seems fine, then.