BUG: MRMLUnitNode: Ensure value to string conversion account for precision

Fixes 0004044

By calling "strstream.setf(ios::fixed,ios::floatfield)", it guarantees
that the returned string will have a number of decimals equal to the
precision (instead of a number of digit equal to precision).

This prevents the cut of decimals when we have big numbers.

Running the following python snippet in the python interactor
before and after integrating this commit illustrates the problem
and confirm it is fixed.

8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---
import math
l = slicer.modules.units.logic()
s = l.GetUnitsScene()
n = s.GetNodesByName("Metre per second").GetItemAsObject(0)
n.SetPrecision(3)
for v in range(-5, 5):
print("10^%d -> %s" % (v, n.GetDisplayStringFromValue(math.pow(10, v))))
8<---8<---8<---8<---8<---8<---8<---8<---8<---8<---

Before the change:

10^-5 -> 1e-05 m/s
10^-4 -> 0.0001 m/s
10^-3 -> 0.001 m/s
10^-2 -> 0.01 m/s
10^-1 -> 0.1 m/s
10^0 -> 1 m/s
10^1 -> 10 m/s
10^2 -> 100 m/s
10^3 -> 1e+03 m/s
10^4 -> 1e+04 m/s

After the change:

10^-5 -> 0.000 m/s
10^-4 -> 0.000 m/s
10^-3 -> 0.001 m/s
10^-2 -> 0.010 m/s
10^-1 -> 0.100 m/s
10^0 -> 1.000 m/s
10^1 -> 10.000 m/s
10^2 -> 100.000 m/s
10^3 -> 1000.000 m/s
10^4 -> 10000.000 m/s

Co-authored-by: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>

From: Davide Punzo <punzodavide@hotmail.it>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24541 3bd1e089-480b-0410-8dfb-8563597acbee