mod2c version and CMake improvement

[pramodk]

Fix version detection issue from CoreNEURON Findmod2c.cmake

• it seems like version specification in CMakeList.txt not considered in mod2c
• mod2c has hardcoded version (?) :

mod2c_core/nocpout.c:char* nmodl_version_ = "6.2.0";
mod2c_core/nocpout.c:	Sprintf(buf, "\"%s\",\n\"%s\",\n", nmodl_version_, mechname);