Use Python's logging system

Abandon our own home-made one in favor of one that uses the `logging`
package of the standard library. It allows more flexibility and removes
some oddities like the double "Logger" definition and some "global"
assignments.
Make FileCacher.delete raise a KeyError
Make FileCacher.get_size use the local copy
Update ParameterTypes

Add default values and a way to get a JSON description.
Add Watcher
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