[list] Show more metadata information when showing information about datasets

[viklund]

As a user
I want to know a little bit more about a dataset before listing its files
So I can take better decisions for the next step

Description

Currently when listing datasets only the ids of the datasets are displayed. Would be good to also display the total size of a dataset and the number of files it contains.

A/C:
output as in below

Example:

$ sda-cli list --datasets
Dataset      Files    Size
dataset-01     345     50 Gb
dataset-02      12     22 Gb
dataset-03    9821    2.7 Tb

[jbygdell]

The actual command should be: sda-cli list-datasets to clearly show that it is the intent.