Cherry pick RTS/src/DAQ_TPX/tpxGain.h from 6dd0b8e

In order to resolve the error:

```
g++ -m64 -fPIC -pipe -Wall -Woverloaded-virtual -std=c++0x -Wno-long-long -pthread -Wno-deprecated-declarations -O2 -g -falign-loops -falign-jumps -falign-functions -Dsl79_gcc485 -D__ROOT__ -DNEW_DAQ_READER -I. -IStRoot/StTpcHitMaker -IStRoot/RTS/src -IStRoot/RTS/include -IStRoot/RTS/trg/include -I. -IStRoot -I.sl79_gcc485/include -I/opt/software/linux-scientific7-x86_64/gcc-4.8.5/root-5.34.38-l3v6vso6qgojm4l2ctwjojs6trbt4hpn/include -c .sl79_gcc485/OBJ/StRoot/StTpcHitMaker/StTpcRTSHitMaker.cxx -o .sl79_gcc485/OBJ/StRoot/StTpcHitMaker/StTpcRTSHitMaker.o
.sl79_gcc485/OBJ/StRoot/StTpcHitMaker/StTpcRTSHitMaker.cxx: In member function 'virtual Int_t StTpcRTSHitMaker::InitRun(Int_t)':
.sl79_gcc485/OBJ/StRoot/StTpcHitMaker/StTpcRTSHitMaker.cxx:216:33: error: invalid conversion from 'const Char_t* {aka const char*}' to 'char*' [-fpermissive]
     if (gain->from_file(fname, 0) < 0) {
                                 ^
In file included from .sl79_gcc485/OBJ/StRoot/StTpcHitMaker/StTpcRTSHitMaker.cxx:49:0:
StRoot/RTS/src/DAQ_TPX/tpxGain.h:142:6: error:   initializing argument 1 of 'int tpxGain::from_file(char*, int)' [-fpermissive]
  int from_file(char *fname, int sector = 0) ;
      ^
.sl79_gcc485/OBJ/StRoot/StTpcHitMaker/StTpcRTSHitMaker.cxx:150:19: warning: unused variable 'fnameITPC' [-Wunused-variable]
     const Char_t *fnameITPC  = "none";
                   ^
cons: *** [.sl79_gcc485/OBJ/StRoot/StTpcHitMaker/StTpcRTSHitMaker.o] Error 1
cons: errors constructing .sl79_gcc485/OBJ/StRoot/StTpcHitMaker/StTpcRTSHitMaker.o
```

Author: plexoos

StRoot/RTS/src/DAQ_TPX/tpxGain.cxx

@@ -828,7 +828,7 @@ void tpxGain::do_default(int sec)
 	return ;
 }
 
-int tpxGain::from_file(char *fname, int sec)
+int tpxGain::from_file(const char *fname, int sec)
 {
 	FILE *f ;
 	int s, r, p ;
@@ -999,7 +999,7 @@ int tpxGain::from_file(char *fname, int sec)
 }
 
 
-int tpxGain::to_file(char *fname) 
+int tpxGain::to_file(const char *fname) 
 {
 
 	FILE *f ;

StRoot/RTS/src/DAQ_TPX/tpxGain.h

@@ -139,7 +139,7 @@ class tpxGain
 		return (gains[s-1] + r*TPX_MAX_PAD + (p-1)) ;
 	}
 
-	int from_file(char *fname, int sector = 0) ;
+	int from_file(const char *fname, int sector = 0) ;
 
 
 	// [sector 1-24][RDO 1-6][fee-index 0-35]
@@ -193,7 +193,7 @@ class tpxGain
 
 
 	// below used only during calculation
-	int to_file(char *fname) ;
+	int to_file(const char *fname) ;
 
 	void init(int sec=0) ;			// zap's structs; assume all TPX
 	void accum(char *evbuff, int bytes) ;	// parses one RDO's worth of pulser data