ENH: extend command-line options for particleTracks

- can specify format, stride without modifying a dictionary
  (increases flexibility, eases testing)

applications/utilities/postProcessing/lagrangian/particleTracks/createControls.H

@@ -0,0 +1,30 @@
+// Read particleTrackProperties dictionary and extract values from it
+
+const word dictName("particleTrackProperties");
+
+#include "setConstantMeshDictionaryIO.H"
+
+IOdictionary propsDict(dictIO);
+
+const word cloudName(propsDict.get<word>("cloud"));
+
+label sampleFrequency(propsDict.get<label>("sampleFrequency"));
+
+label maxPositions(propsDict.get<label>("maxPositions"));
+
+label maxTracks(propsDict.getOrDefault<label>("maxTracks", -1));
+
+word setFormat(propsDict.getOrDefault<word>("setFormat", "vtk"));
+
+// Optional - if empty, select all
+const wordRes fieldNames(propsDict.getOrDefault<wordRes>("fields", wordRes()));
+
+const word UName(propsDict.getOrDefault<word>("U", "U"));
+
+const dictionary formatOptions
+(
+    propsDict.subOrEmptyDict("formatOptions", keyType::LITERAL)
+);
+
+
+// ************************************************************************* //