ENH: SubStrings::str(int) method for similarity with std::smatch

- define regExp::results_type using SubStrings container for handling
  groups. This makes a later shift to std::smatch easier, but changes
  the regExp API for matching with groups. Previously had list element
  0 for regex group 1, now list element 0 is the entire match and list
  element 1 is regex group 1.

  Old:
      List<std::string> mat;
      if (re.match(text, mat))  Info<< "group 1: " << mat[0] << nl;

  New:
      regExp::results_type mat;
      if (re.match(text, mat))  Info<< "group 1: " << mat.str(1) << nl;

applications/test/regex/testRegexps

@@ -10,27 +10,29 @@
 
 // Pattern, String
 (
-    ( "a.*"  "abc" )        // true
-    ( "a.*"  "bac" )        // false - partial match only
-    ( "a.*"  "abcd" )       // true
-    ( "a.*"  "def" )        // false
-    ( ".*a.*"  "Abc" )      // false
-    ( "(?i).*a.*"  "Abc" )  // true
-    ( "d(.*)f"  "def" )     // true
-    (
+    ( true  "a.*"  "abc" )
+    ( true  "a.*"  "abc" )
+    ( false "a.*"  "bac" )        // partial match only
+    ( true  "a.*"  "abcd" )
+    ( false "a.*"  "def" )
+    ( false ".*a.*"  "Abc" )
+    ( true  "(?i).*a.*"  "Abc" )
+    ( true  "d(.*)([xy]*)([f-p])"  "def" )
+    ( false "d(.*)([xy]*)([f-p])"  "xxdef" )
+    ( true
         " *([A-Za-z]+) *= *([^ /]+) *(//.*)?"
         "   keyword  = value // comment"
-    )   // true
-
+    )
     (
+        false   // partial match only
         "[[:digit:]]"
         "contains 1 or more digits"
-    )   // false - partial match only
-
+    )
     (
+        true
         "[[:digit:]]+-[[:digit:]]+-[[:digit:]]+-[[:digit:]]+"
         "1-905-123-2234"
-    )   // true
+    )
 
 )