The dense drag models (WenYu, ErgunWenYu and PlessisMasliyah) have been
extended so that they can create their own void-fraction field when one
is not otherwise available.
The duplicated functionality in the drag models has been generalised and
is now utilised by multiple models.
References have been added for all models for which they could be found.
In addition, an additional drag model, SchillerNaumann, has been added.
This is of the same form as sphereDrag, and is designed for the same
scenario; sparsely distributed spherical particles. The SchillerNaumann
form is more widely referenced and should now be considered standard.
The thermo-parcel-specific force instantiation macro has been removed,
as all the force models are now possible to use with all parcel types.
Resolves bug report https://bugs.openfoam.org/view.php?id=3191
For multi-region cases the default location of blockMeshDict is now system/<region name>
If the blockMeshDict is not found in system then the constant directory
is also checked providing backward-compatibility
