fix `get_array`, add wrappers to some of `extra_data` basic functions

While addressing #25 (closed), I tried to make get_array a function that can be seen as a wrapper for DataCollection.get_array but using the ToolBox mnemonics, and for which one can also select a subset. It can be a quick way of loading 1 mnemonic at a time without using load.

I also added open_run as a wrapper to extra_data.open_run() and find_run_path, for convenience. This cleans a bit the beginning of load.

@lleguy