Handle observation standard error files with gen_obs et get_obs

[sbuis]

The gen_obs and get_obs functions generate and read files named "usm_name.obs". This is adapted to handle observation files but does not work for observation standard error files, although they have the same structure as the observation files and can be used in CroPlotR plot function.
An additional argument (that would just control the files extension: ".obs" or ".sd") could be easily added to these functions to handle these files without any big change.