Add zip file format
Add magic word when summarize filesystem based caches
Fix typo
Add cluster storage for large scale rendering (not throughly tested yet).
Move transtorage.py to scripts directory.
Add shortcut to check MetaTile existence so renderer can continue from last interrupted render even if no MetaTileStorage is used.
Make overwrite option work again...
Tested cluster storage and fix various bugs.
Disable range check in has_metatile
Fix create dir race condition in __init__()

Up to now this problem only occurs on mounted NFS volume on ubuntu...
Use bilinear sampling method when zoom ratio matches closely
Fix 'grid' artifact on ned10m data @ zoom level 13
add config 'keep_cache' for cache control
Merge branch 'feature/cluster-storage' of
git@github.com:Kotaimen/mason.git into feature/cluster-storage
Merge branch 'feature/cluster-storage' into develop