Enable resetting time signature in staff construction

core/src/main/kotlin/io/smnp/evaluation/evaluator/StaffEvaluator.kt

@@ -64,7 +64,9 @@ class StaffEvaluator : Evaluator() {
       val evaluatedSignature = calculateTimeSignature(evaluatedMeasure)
 
       currentSignature?.let {
-         if (evaluatedSignature != it) {
+         when {
+            it.numerator == 0 -> return
+            evaluatedSignature != it -> {
                val simplified = evaluatedSignature.simplified
                throw contextEvaluationException(
                   "Invalid time signature: expected ${it.numerator}/${it.denominator}, got ${simplified.numerator}/${simplified.denominator}",
@@ -73,6 +75,9 @@ class StaffEvaluator : Evaluator() {
                )
             }
          }
+
+         null
+      }
    }
 
    private fun calculateTimeSignature(values: List<Value>): Fraction {