Write interpolation weight into a file

[mhnguyen]

XIOS is able to read weights from a file then use them to do interpolation of a domain to another.
There are cases in which these weights should be computed once then reused over and over.
Therefore, XIOS should be able to write weights into file.
Some specifications for this:

• Flag for interpolation_domain

+) filename: Already exists
+) weights="read/write"

• Behaviors:

+) weights="write" -> write weights into file. If there is no filename -> default filename weights.nc will be used, otherwise use filename. Filename will be always overwritten in this case.
+) weights="read" -> try to read weights from a file. If there is no filename-> default filename weights.nc will be used, otherwise use filename. If there is no file to read at all-> switch to write mode.

[ymipsl]

Done