modelviz: filter out modules without groups to avoid NPE error

These modules without groups are injected one or implicit bottom modules.

Now `nitmetrics m1.nit m2.nit --all` do not crash.

Signed-off-by: Jean Privat <jean@pryen.org>
nitmetrics: do not crash in `to_console` if no values to present

The crash occurs since `max` and `min` assert that `not values.is_empty`.

Signed-off-by: Jean Privat <jean@pryen.org>
tests: update nitmetrics stuff

Note: nobody saw it but there was a `Assert failed` in the previous sav file.

Signed-off-by: Jean Privat <jean@pryen.org>
model: fix module owner placebo

nitdoc still rely on the old nesting hierarchy.
But the implementation of the placebo was buggy if a nested module is
loaded before its nesting module (because a wrongful usage of
`.mmodules.first`).

This patch has two small parts

* add a method `fuzzy_owner` to a MGroup (to replace the crazy
  `.mmodules.first`)
* a special case is added when a nesting module is loaded after some of
  its nested ones

Fix #324
Signed-off-by: Jean Privat <jean@pryen.org>
Merge branch 'fix_module_owner_placebo'
Merge branch 'metrics'