nrnivmodl-core does not accept individual mod files as input

[JCGoran]

While calling nrnivmodl [mod1] [mod2] ... works, where [mod1] etc. are individual mod files, calling nrnivmodl-core [mod1] [mod2] ... does not, i.e. nrnivmodl-core only seems to accept a single directory that contains mod files.
This also affects nrnivmodl -coreneuron [mod1] [mod2] (because it basically just calls nrnivmodl-core), and it would be beneficial to have consistent behavior between the two.

[pramodk]

@alkino : maybe this is simple enough to fix? At least nrnivmodl -coreneuron [mod] would be nice to have working.