functionObjects: renamed faceSource -> surfaceRegion, cellSource -> volRegion

The use of the term 'source' in the context of post-processing is
confusing and does not properly describe the process of region
selection.  The new names 'surfaceRegion' and 'volRegion' better
describe the purpose of the functionObjects which is to provide field
processing functionality limited to a specified region of space, either
a surface or volume.

The keyword 'source' is renamed 'regionType' which better describes the
purpose which is to specify the method by which the surface or volume
region is selected.

The keyword to select the name of the surface or volume region is
renamed from 'sourceName' to 'name' consistent with the other
name-changes above.