Don't automatically set instantiate to True when constant is True

[maximlt]

Fixes #287

Pushing this WIP PR to get some feedback on the approach. The idea is that when:

1. instantiate is True -> deepcopy default and assign it to the local cache (pseudo: parameterized_obj._X_param_value = deepcopy(parameter.default))
2. constant is True -> don't deepcopy, just assign a reference to default to the local cache (pseudo: parameterized_obj._X_param_value = parameter.default)

With 1. taking precedence over 2.

The test added in 7635d55 and that is based on the one given in the issue passes with this change.

Of course this means that if the default object is mutable, any change made from a reference will be seen by all the other references, all pointing to the same updated object.

import param

class A(param.Parameterized):
    x = param.Number(0)

a = A(name="AAA")

class B(param.Parameterized):
    y = param.ObjectSelector(a, constant=True)

b = B(name="BBB")

B.y.x = 1
print(b.y.x)
# 0 before this PR
# 1 with this PR

b.y.x = 2
print(B.y.x)
# 1 before this PR
# 2 with this PR

If reviewers are OK with this approach, I'll need to clean up the code and do some refactoring, it's a bit hacky right now.

[philippjfr]

This looks good to me. It's always very difficult to consider all the implications of these changes but let's just make sure we run all test suites against this and then merge.

[maximlt]

The unit test suites of Panel, HoloViews and Lumen passed with this branch checked out. Merging.