Constants like FARADAY, gasconstant are defined in multiple sources

[pramodk]

NMODL translated MOD files get unit constants typically from share/lib/nrnunits.lib.in. But there are other source files that hardcode some of the constants. For example:

→ grep -r -n FARADAY nn/src/*
nn/src/nmodl/version.c:624: * UNITS { FARADAY = 96520 (coul) }
nn/src/nrniv/kschan.h:74:// e/(kT) e/k=11.604589 from hoc's FARADAY and R values
nn/src/nrnoc/eion.c:218:#define FARADAY 96485.309
nn/src/nrnoc/eion.c:221:#define FARADAY 96485.33289
nn/src/nrnoc/eion.c:225:#define ktf (1000.*gasconstant*(celsius + 273.15)/FARADAY)
nn/src/nrnoc/eion.c:302:	return (.001)*z*FARADAY*(eci - eco);
nn/src/oc/hoc_init.c:93:	"FARADAY", 96485.309,	/*coulombs/mole*/
nn/src/oc/hoc_init.c:97:	/* also see FARADAY and gasconstant in ../nrnoc/eion.c */
nn/src/oc/hoc_init.c:98:	"FARADAY", 96485.33289,	/*coulombs/mole*/
nn/src/oc/version.c:715: * FARADAY and R (molar gas constant) added as built-in constants.

This also applied to CoreNEURON. We should uniform these definitions into single location to be consistent.