utilities/exceptions: Drop 'UndefinedReferenceWarning'

This warning had the function to inform the user if she/he/* used a
tempo point without a reference. Then this reference was automatically
set to 1. Unfortunately this warning isn't very useful. Very often such
tempo points are used and then the warnings just pollute the logging.
They may also look frightening to new users, while they can simply be
ignored. So it's better to drop this warning.

mutwo/core_converters/tempos.py

@@ -58,7 +58,6 @@ def _extract_beats_per_minute_and_reference_from_tempo_point(
         try:
             reference = tempo_point.reference  # type: ignore
         except AttributeError:
-            self._logger.warning(core_utilities.UndefinedReferenceWarning(tempo_point))
             reference = 1
 
         return beats_per_minute, reference

mutwo/core_utilities/exceptions.py

@@ -1,9 +1,5 @@
 """Module for mutwo specific exceptions."""
 
-import typing
-
-from mutwo import core_constants
-
 __all__ = (
     "CannotSetDurationOfEmptyComplexEvent",
     "AlreadyDefinedValueNameError",
@@ -19,7 +15,6 @@
     "InvalidCutOutStartAndEndValuesError",
     "SplitUnavailableChildError",
     "EmptyEnvelopeError",
-    "UndefinedReferenceWarning",
     "ConcatenationError",
     "NoTagError",
     "SplitError",
@@ -139,12 +134,12 @@ def __init__(self, start, end, simple_event, duration):
 
 
 class SplitError(Exception):
-    def __init__(self, absolute_time: 'core_parameters.abc.Duration'):
+    def __init__(self, absolute_time: "core_parameters.abc.Duration"):
         super().__init__(f"Can't split event at absolute time '{absolute_time}'.")
 
 
 class SplitUnavailableChildError(Exception):
-    def __init__(self, absolute_time: 'core_parameters.abc.Duration'):
+    def __init__(self, absolute_time: "core_parameters.abc.Duration"):
         super().__init__(
             f"Can't split child at absolute time '{absolute_time}'. There is no child"
             " event available at the requested time."
@@ -156,15 +151,6 @@ def __init__(self, envelope, method):
         super().__init__(f"Can't call '{method}' on empty envelope '{envelope}'!")
 
 
-class UndefinedReferenceWarning(RuntimeWarning):
-    def __init__(self, tempo_point: typing.Any):
-        super().__init__(
-            f"Tempo point '{tempo_point}' of type '{type(tempo_point)}' "
-            "doesn't know attribute 'reference'."
-            " Therefore reference has been set to 1."
-        )
-
-
 class ConcatenationError(TypeError):
     def __init__(self, ancestor, event):
         super().__init__(