Allow callable (dynamic) values for Date and CalendarDate

[gaoflow]

Date and CalendarDate subclass Number, and all of them ultimately subclass Dynamic — whose contract is that a parameter may be set to a callable that is invoked to produce the value on get:

import datetime, param

class P(param.Parameterized):
    n = param.Number()
    d = param.Date()

p = P()
p.n = lambda: 42          # fine
p.d = lambda: datetime.datetime.now()
# ValueError: Date parameter 'P.d' only takes datetime and date types, not <class 'function'>

Number._validate_value explicitly allows a callable (callable(value) and not inspect.isgeneratorfunction(value)), and Integer inherits it — but the Date and CalendarDate overrides drop that guard and go straight to the datetime type check, so they reject a callable that their sibling parameters accept.

The dynamic machinery itself already handles callable dates correctly — with the guard added, p.d resolves to the datetime and re-evaluates on each access like any other Dynamic value; only _validate_value was rejecting them.

Fix. Add the same callable guard Number uses to Date._validate_value and CalendarDate._validate_value, so all Dynamic siblings behave uniformly.

Test. TestDynamicCallableAcrossNumberSiblings (in tests/testdynamicparams.py) parametrizes over Number, Integer, Date, CalendarDate and asserts each accepts a callable and resolves it on get, both via assignment and via constructor default=. Full suite (testdynamicparams, testdateparam, testcalendardateparam, testnumberparameter): 104 passed.