Add bounds for vertical axis

[ymipsl]

Add an optionnal attribute for vertical axis named "bounds" of double array.
The dimensions must be [n][2] where n is the size of the axis.
If this attribute is present, the bounds must be writen in the nectdf file.
For example :
If the name of the verical axis is "presniv" :
we must have in the file:
dimensions:

presnivs = 64 ;
presnivs_nbounds=2 ;
float presnivs(presnivs) ;

presnivs:units = "Pa" ;
presnivs: bounds = "presnivs_bounds"

variable:

float presnivs_bounds(presnivs,presnivs_nbounds) ;

This must be also reported into XIOS-2.0

[rlacroix]

Fixed in r609.