STYLE: use string substr instead of string::operator()

- makes the purpose clearer.
  In some places, string::resize() is even simpler.

- use C++11 string::back() in preference to str[str.size()-1]

src/meshTools/edgeMesh/edgeMeshFormats/nas/NASedgeFormat.C

@@ -74,7 +74,7 @@ bool Foam::fileFormats::NASedgeFormat::read
         // Check if character 72 is continuation
         if (line.size() > 72 && line[72] == '+')
         {
-            line = line.substr(0, 72);
+            line.resize(72);
 
             while (true)
             {
@@ -87,7 +87,7 @@ bool Foam::fileFormats::NASedgeFormat::read
                 }
                 else
                 {
-                    line += buf.substr(8, buf.size()-8);
+                    line += buf.substr(8);
                     break;
                 }
             }

src/meshTools/edgeMesh/edgeMeshFormats/obj/OBJedgeFormat.C

@@ -117,9 +117,9 @@ bool Foam::fileFormats::OBJedgeFormat::read(const fileName& filename)
         string line = this->getLineNoComment(is);
 
         // handle continuations
-        if (line[line.size()-1] == '\\')
+        if (line.back() == '\\')
         {
-            line.substr(0, line.size()-1);
+            line.resize(line.size()-1);
             line += this->getLineNoComment(is);
         }