support/graph-size: display human-readable size

support/graph-size: display human-readable size

Currently, we forcibly report sizes in multiple of Kilobytes. In some
big configurations, the sizes of the system as a whole, as well as that
of individual packages, may exceed megabytes, and when some artistic
assets get used, even the gigabyte may get exceed.

These big sizes are not easy to read when expressed in kilobytes.

Additionally, some very small packages might have sizes below the
kilobyte (and when we can specify the cut-off grouping size, they may
get reported), and thus the size displayed for those would be 0 kB.

Add a helper function that can format a floating-point size into a
string with all the appropriate formatting:

  - there are at least 3 meaningfull digits visible, i.e. we display
    "3.14" or "10.4" instead of just "3" or "10", but for big number we
    don't care about too many precision either, so we report "100" or
    "1000", not "100.42" or "1000.27";

  - the proper SI prefix is appended, if needed.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>