GEOSldas should add ntasks-per-node as an optional input

[mathomp4]

In #106, the use of ntasks-per-node as a required argument was removed. And for most runs this is good, as it makes things more general.

But, @biljanaorescanin found a need to return it as an optional argument. She was running the LDAS at c2160. In an older job, she was using like 1260 cores and 80 cores-per-node because the job needed more memory per core.

However, currently, there is not way in the input file to specify this, so a user must do it after the lenkf.j file is made. If you don't, the job is killed by NCCS for using too much memory on a node.

Looking at ldas_setup, there seem to be both required and optional arguments, and I think ntasks-per-node is a viable optional argument.

cc @weiyuan-jiang @gmao-rreichle

[gmao-rreichle]

Sorry this got lost in the shuffle. For now, @biljanaorescanin, just edit lenkf.j with the desired SLURM directives. We'll sort out how to refine the processing of the SLURM directives through ldas_setup as soon as we can.