Support star-over on process iterruption

When the processing is interrupted, we shall be:

• in a clean state, i.e. incomplete files shall not be available in the directory of finalized products,
• able to start over, and ignore products that have already been generated -- relates to #22 (closed).

Envisioned technical solutions

1. Regarding incomplete files:

   1. OTB application will produce file in a temporary folder
   2. on successful completion, the temp file will be moved to its final folder

   Question: How can we make sure to clean automatically clean temporary files from a Python process that has been interrupted??

2. Regarding starting-over

   1. On process start-up: scan the list of available products to know what already exists
   2. When an OTB application successfullly completes its job, update the internal list

   That list won't be persisted in between executions of S1Tiling.