xvalues_t:
=========
This typedef represents a xValue<VT> type with VT the template parameter
corresponding to the type of value stored in xValue.
It is introduced to hopefully avoid misunderstanding with previously
introduces value_t corresponding in most case to the type of value
computed (for now we play mostly with arithmetic with those).

Especially in xValueLinearCombinationXXX hierarchy we are dealing
with xvalues_t, not value_t.

It has been also introduced in xValueManagerDist to clearly separate the
type of the values, value_t, from the type of the xValue<value_t> that
store in the xValManager instance these information.

This as been used as suggested by N.C. to set the type of the value_container_t
and be more strict regarding the implementation

value_manager_t:
===============
In xField value_manager_t replace ValueManager to try to clarify
situation.

getValueManager:
================
getValueManager and getDoubleManager are now rigorously the same. So this
commit removes this last method that do have a name far to specific.

This implies lost of changes everywhere ... but re-naming of instance have
not been done accordingly to this new method name (to much work). Names
such as double_manager or dm should be changed ... little by little

Test will be updated soon.