add pyyaml to requirements for yaml definition of pipelines

Author: ahoermann

requirements.txt

@@ -10,3 +10,4 @@ stamina
 tox
 scipy
 uncertainties
+pyyaml

src/modacor/runner/pipeline.py

@@ -4,6 +4,7 @@
 
 from graphlib import TopologicalSorter
 from pathlib import Path
+import yaml
 
 from attrs import define, field
 from attrs import validators as v