weather-sp: Provide an option to append the filename with the splitted filename.

Currently, **weather-sp** provides an option to set the filename of the splitted file.

For example, for the input file `--input-pattern 'gs://bucket-name/2017-1-sp-10u-10v-2t-2d-tp-ptype.gb'`, I provided `--output-template 'gs://bucket-name/monthly-splitted/{date}{time}/{step}/{date}{time}_FH-{step}.gb'` and it created splitted files like `gs://bucket-name/monthly-splitted/201201010000/0/201201010000_FH-0.gb`.

This input file is for a few variables. Now in the future, when a new raw file for a new variable comes for the same timestamp, I cannot split that file in the monthly-splitted folder, because it will override the files as they just includes date, time and step. What I really want is an option to have a functionality to append the raw data filename to the splitted data filename. Somewhat like ability to provide that in the --output-template argument only like `--output-template 'gs://bucket-name/monthly-splitted/{date}{time}/{step}/{filename}_{date}{time}_FH-{step}.gb'`