Develop testeleftheria

Hi,

I added a new function (vertmeanvar) to compute weighted vertical means in the ocean, on either temperature or salinity (potentially on every quantity on gridT). The input levels should be in level number, and they are specified in the configure file with the 2 new variables level1 and level2 (the modified config file is part of the commit). People who would be interested in this and could check it: @vguemas  @cprodhomme  @pabretonniere

Cheers
Eleftheria