more configuration for FileCachedFunction

The current handling of configuration file for FileCachedFunction is very fragile. If two sage sessions are open at the same time and performs configuration saving one can erase the other. To fix it, we make the configuration "per function" rather than "per directory". Note that this is a backward incompatible change. Though it does fix a bug which would be painful to avoid with the previous setting.

We also make available more configuration options, namely cache_read, cache_write, file_read, file_write and online_lookup so that one can completely deactivate the cache features by setting all of them to False. We perform the latter operation in the benchmarks.

Additionally, we include file_cache.py in the documentation.