fix disjoint qubit layering (#247)

## Description

Currently, the layering strategy `DisjointQubits` always inserts gates
in the last layer. By instead inserting it one layer after the first
layer (from the back of the circuit), in which it would intersect with
the qubit set of a previous gate, we achieve a still valid layering with
less layers. Visually speaking, we move each gate as far left as
possible without intersecting other gates.

The following visualizations illustrate the status-quo and the new
solution proposed in this PR on the following circuit:

![circuit_ordered](https://user-images.githubusercontent.com/15572450/221338578-3e66bd61-0e3c-466d-9227-53f4b43a56ac.png)
The numbers next to the gates indicate the gate order in the circuit.

Currently, the following layering strategy is implemented (red dots
indicate the state of `laystLayer` for each qubit after inserting the
gate):

![current](https://user-images.githubusercontent.com/15572450/221338498-8a1c785e-d9b6-4938-8e93-8445ea870636.png)

The proposed implementation:

![lukas](https://user-images.githubusercontent.com/15572450/221338519-bbe2b7aa-de98-4dc4-a6e2-effb25e60e16.png)

## Checklist:

- [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.

src/Mapper.cpp

@@ -42,6 +42,7 @@ void Mapper::processDisjointQubitLayer(
     } else {
       layer = *lastLayer.at(target) + 1;
     }
+    lastLayer.at(target) = layer;
   } else {
     if (!lastLayer.at(*control).has_value() &&
         !lastLayer.at(target).has_value()) {
@@ -53,20 +54,17 @@ void Mapper::processDisjointQubitLayer(
     } else {
       layer = std::max(*lastLayer.at(*control), *lastLayer.at(target)) + 1;
     }
+    lastLayer.at(*control) = layer;
+    lastLayer.at(target)   = layer;
   }
 
   if (layers.size() <= layer) {
     layers.emplace_back();
-  } else {
-    layer = layers.size() - 1;
   }
   if (control.has_value()) {
-    layers.back().emplace_back(*control, target, gate);
-    lastLayer.at(*control) = layer;
-    lastLayer.at(target)   = layer;
+    layers.at(layer).emplace_back(*control, target, gate);
   } else {
-    layers.back().emplace_back(-1, target, gate);
-    lastLayer.at(target) = layer;
+    layers.at(layer).emplace_back(-1, target, gate);
   }
 }