reorder backend methods and raise error when backend is read-only

[vincentsarago]

This PR does:

• reorder methods within backends for better visibility
• remove mosaic_def in __init__ for read-only backends

with HttpBackend(url, mosaic_def=mosaic): 
     pass

☝️ this will now raise a TypeError with mosaic_def not in init method.

• add minzoom/maxzoom/bounds default for the backends (those are mandatory for the BaseReader but we never really set them. They are not set from the mosaicjson doc)