3 new Library functions (compute_l2a, compute_cloudmasks, compute_indices) to multi-process a list of tiles

updated indices functions to use cm001 as default, utilization of other cloud masks not implemented yet

Multiple corrections according to new nomenclature and multiple CM versions - Introducing Sen2Chain 0.7.0

L2A, cloudmasks and indices versions OK, new NDRE indice, preparing new IRECI vegetation index and simpler indice declaration

bug fixes: 1. removed -i option in sen2cor bash init command stopping processes while chaining two images 2. L2A 20m cloud mask paths were not correctily retrieved when 60m processing was enabled

introducing new functions to manage Tile and Library and new TileSet class to compute l2a, cloudmasks and indices for multiple Tiles using multiprocessing"