parameters.abc += Parameter

In 56c7480 I already added an abstract base class for the
parameter type. But in 1bd0c31 this class was removed again.
There was a discussion in #11 if it makes sense
to have an empty class.

Nowadays we can assume that it *does* make sense to do so:

- after the introduction of 'MutwoObject' all parameters should inherit
  from this: by using a common parent class this necessity can be
  simplified and be more reliable

- for type hints, in case we really do only accept mutwo parameters (and
  not any object as it's often ok too), we can use the 'abc.Parameter'
  class. In mutwo.core < 2.0.0 we used to have
  'core_constants.ParameterType', but this rather made the situation
  more complicated. By simply using either 'typing.Any' or 'abc.Parameter'
  it's clear what is expected

mutwo/core_parameters/abc.py

@@ -32,6 +32,7 @@
 from mutwo import core_utilities
 
 __all__ = (
+    "Parameter",
     "SingleValueParameter",
     "SingleNumberParameter",
     "ParameterWithEnvelope",
@@ -40,7 +41,16 @@
 )
 
 
-class ParameterWithEnvelope(core_utilities.MutwoObject, abc.ABC):
+class Parameter(core_utilities.MutwoObject, abc.ABC):
+    """A Parameter is the base class for all mutwo parameters
+
+    It can be useful as a type hint for any code where a parameter
+    object is expected. This isn't necessarily at many places,
+    as mostly any object can be assigned as a parameter to an event.
+    """
+
+
+class ParameterWithEnvelope(Parameter):
     """Abstract base class for all parameters with an envelope."""
 
     def __init__(self, envelope: core_events.RelativeEnvelope):
@@ -75,7 +85,7 @@ def resolve_envelope(
         return self.envelope.resolve(duration, self, resolve_envelope_class)
 
 
-class SingleValueParameter(core_utilities.MutwoObject, abc.ABC):
+class SingleValueParameter(Parameter):
     """Abstract base class for all parameters which are defined by one value.
 
     Classes which inherit from this base class have
@@ -318,7 +328,7 @@ def duration(self, duration: core_constants.Real):
 DurationOrReal = Duration | core_constants.Real
 
 
-class TempoPoint(core_utilities.MutwoObject, abc.ABC):
+class TempoPoint(Parameter):
     """Represent the active tempo at a specific moment in time.
 
     If the user wants to define a `TempoPoint` class, the abstract