Implement a new type of callback called validators, which get called before a value changes

[astrofrog]

This is an experimental pull request adding a new type of callback called 'validators'. The idea here is that one can do e.g.:

In [1]: from echo import HasCallbackProperties, CallbackProperty, ValidationException, SilentValidationException
   ...: 
   ...: class State(HasCallbackProperties):
   ...: 
   ...:     a = CallbackProperty()
   ...:     b = CallbackProperty()
   ...: 
   ...: state = State()
   ...: state.a = 1
   ...: state.b = 2.2
   ...: 

In [2]: def add_one_and_silent_ignore(new_value):
   ...:     if new_value == 'ignore':
   ...:         raise SilentValidationException()
   ...:     return new_value + 1
   ...: 
   ...: def preserve_type(old_value, new_value):
   ...:     if type(new_value) is not type(old_value):
   ...:         raise ValidationException('types should not change')
   ...: 
   ...: state.add_callback('a', add_one_and_silent_ignore, validator=True)
   ...: state.add_callback('b', preserve_type, validator=True, echo_old=True)
   ...: 
   ...: 

In [3]: state.a = 3

In [4]: state.a
Out[4]: 4

In [5]: state.a = 'ignore'

In [6]: state.a
Out[6]: 4

In [7]: state.b = 3.2

In [8]: state.b = 3
---------------------------------------------------------------------------
ValidationException                       Traceback (most recent call last)
...
ValidationException: types should not change

This demonstrates several features:

• Checks can be carried out on the type and value of the new value (and the old one if echo_old is passed)
• Validators can raise either full exceptions (ValidationException) or SilentValidationException, the latter of which means that the change will be silently abandonned
• Validators can change the value to be set, either in-place or by returning a new object

[codecov]

Codecov Report

Attention: Patch coverage is 98.71795% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.81%. Comparing base (d4d9d2f) to head (b0662e5).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
echo/core.py	96.15%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #42      +/-   ##
==========================================
+ Coverage   96.70%   96.81%   +0.11%     
==========================================
  Files          18       18              
  Lines        2274     2327      +53     
==========================================
+ Hits         2199     2253      +54     
+ Misses         75       74       -1     

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