Avoid constant/polyMesh/blockMeshDict

Finally possible to use system/blockMeshDict. Avoid using constant/polyMesh/blockMeshDict since it may now get cleaned out accidentally.

Mentioned in commit e3e700c0

@Prashant - as previously discussed. I think that I have caught all of them.

Thanks.

Probably this should make a place in release documentation (@matej) as caution for users.

Maybe we can have a little blog entry - How to upgrade from version 2.3.1 and below to version 1606+.

Mentioned in commit f850a5aa

closed

mentioned in issue #900 (closed)