🐛 Fix PiExpression division and multiplication arithmetic (#549)

## Description

Fix Arithmetic Issues when dividing or multiplying a `PiExpression` by a
`double`.

Partially solves #486

## Checklist:

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- [x] The pull request only contains commits that are related to it.
- [x] I have added appropriate tests and documentation.
- [x] I have made sure that all CI jobs on GitHub pass.
- [x] The pull request introduces no new warnings and follows the
project's style guidelines.

include/mqt-core/operations/Expression.hpp

@@ -252,7 +252,7 @@ class Expression {
       return *this;
     }
     std::for_each(terms.begin(), terms.end(), [&](auto& term) { term *= rhs; });
-    constant *= U{rhs};
+    constant = U{double{constant} * double{rhs}};
     return *this;
   }
 
@@ -275,7 +275,7 @@ class Expression {
       throw std::runtime_error("Trying to divide expression by 0!");
     }
     std::for_each(terms.begin(), terms.end(), [&](auto& term) { term /= rhs; });
-    constant /= U{rhs};
+    constant = U{double{constant} / double{rhs}};
     return *this;
   }
 

test/zx/test_expression.cpp

@@ -196,3 +196,15 @@ TEST_F(ExpressionTest, Instantiation) {
   EXPECT_THROW([[maybe_unused]] const auto h = e.evaluate(assignment),
                sym::SymbolicException);
 }
+
+TEST_F(ExpressionTest, Arithmetic) {
+  const auto beta = zx::PiRational(1, 2);
+
+  EXPECT_EQ(beta * 2, zx::PiRational(1, 1));
+  EXPECT_EQ(beta * 2., zx::PiRational(1, 1));
+
+  const auto betaExpr = zx::PiExpression{beta};
+  EXPECT_EQ(betaExpr * 2., zx::PiExpression{zx::PiRational(1, 1)});
+
+  EXPECT_EQ(betaExpr / .5, zx::PiExpression{zx::PiRational(1, 1)});
+}