add resolver and tests

[cwebear]

resolver.py carry abstract. class FileResolver returning a list of files to use for the currrent execution
and two children classes

• ExplicitFiles which is only the simplest resolver returning explicit files from list or prom a parent path.
• PathTemplate, a more complexe resolver returning files following a pattern of file naming and a file tree given by the user. useful for large files repositories like ERA5 stored on disk locally. Can return dedicated files from pointset positions + extended temporal window