BUG: Explicitly set keep particle flag in Colliding and MPPIC clouds

This stops all the particles on a given processor being deleted if the
last particle considered in the previous step was not being kept.

src/lagrangian/intermediate/parcels/Templates/CollidingParcel/CollidingParcel.C

@@ -80,6 +80,8 @@ bool Foam::CollidingParcel<ParcelType>::move
 
             p.angularMomentum() += 0.5*trackTime*p.torque();
 
+            td.keepParticle = true;
+
             break;
         }
 

src/lagrangian/intermediate/parcels/Templates/MPPICParcel/MPPICParcel.C

@@ -76,6 +76,8 @@ bool Foam::MPPICParcel<ParcelType>::move
             p.UCorrect() =
                 td.cloud().dampingModel().velocityCorrection(p, trackTime);
 
+            td.keepParticle = true;
+
             break;
         }
         case TrackData::tpPackingNoTrack:
@@ -83,6 +85,8 @@ bool Foam::MPPICParcel<ParcelType>::move
             p.UCorrect() =
                 td.cloud().packingModel().velocityCorrection(p, trackTime);
 
+            td.keepParticle = true;
+
             break;
         }
         case TrackData::tpCorrectTrack: