OSspecific: altered file tests

- removed the previously added fileName type(), isDir(), isFile() methods.

- added optional bool parameter to isFile() function to explicitly
  enable/disable the check for gzip files.

- fixed minor bugginess where the default usage of isFile() would result in
  false positive matches.

- be slightly more stringent and use isDir() and isFile() instead of
  exists() function when we actually know the expected type.

src/sampling/sampledSurface/writers/dx/dxSurfaceWriter.C

@@ -302,7 +302,7 @@ void Foam::dxSurfaceWriter<Type>::write
 {
     fileName surfaceDir(samplePath/timeDir);
 
-    if (!exists(surfaceDir))
+    if (!isDir(surfaceDir))
     {
         mkDir(surfaceDir);
     }

src/sampling/sampledSurface/writers/foamFile/foamFileSurfaceWriter.C

@@ -63,7 +63,7 @@ void Foam::foamFileSurfaceWriter<Type>::write
 {
     fileName surfaceDir(samplePath/timeDir/surfaceName);
 
-    if (!exists(surfaceDir))
+    if (!isDir(surfaceDir))
     {
         mkDir(surfaceDir);
     }
@@ -84,7 +84,7 @@ void Foam::foamFileSurfaceWriter<Type>::write
     fileName foamName(pTraits<Type>::typeName);
     fileName valuesDir(surfaceDir  / (foamName + Field<Type>::typeName));
 
-    if (!exists(valuesDir))
+    if (!isDir(valuesDir))
     {
         mkDir(valuesDir);
     }

src/sampling/sampledSurface/writers/raw/rawSurfaceWriter.C

@@ -342,7 +342,7 @@ void Foam::rawSurfaceWriter<Type>::write
 {
     fileName surfaceDir(samplePath/timeDir);
 
-    if (!exists(surfaceDir))
+    if (!isDir(surfaceDir))
     {
         mkDir(surfaceDir);
     }

src/sampling/sampledSurface/writers/stl/stlSurfaceWriter.C

@@ -64,7 +64,7 @@ void Foam::stlSurfaceWriter<Type>::write
 {
     fileName surfaceDir(samplePath/timeDir);
 
-    if (!exists(surfaceDir))
+    if (!isDir(surfaceDir))
     {
         mkDir(surfaceDir);
     }

src/sampling/sampledSurface/writers/vtk/vtkSurfaceWriter.C

@@ -309,7 +309,7 @@ void Foam::vtkSurfaceWriter<Type>::write
 {
     fileName surfaceDir(samplePath/timeDir);
 
-    if (!exists(surfaceDir))
+    if (!isDir(surfaceDir))
     {
         mkDir(surfaceDir);
     }