ENH: lagrangian injection - exposed minimum number of particles per parcel. See #728

The minimum number of particles per parcel can now be set in the
injection model input, e.g.:

    model1
    {
        type            ...;
        massTotal       ...;
        parcelBasisType ...;
        minParticlesPerParcel 1; <-- new optional entry
        SOI             ...;
        ...

Uses a value of 1 by default if the entry is not present.  The value of
1 is generally recommended and beneficial for coupled cases where small
time steps can lead to the injection of too many parcels and subsequently
greatly over-predict the particle source contributions (momentum, heat,
mass transfer etc)