Make string_to_num work with floats.

It basically amounts to a name change, because the functionality
was already there. There's also a new test for it.

Change-Id: I982ed9de87862bd65b29a81be858b45040bc9d45
diff --git a/aos/linux_code/logging/log_displayer.cc b/aos/linux_code/logging/log_displayer.cc
index 7da0dc4..b2f4d45 100644
--- a/aos/linux_code/logging/log_displayer.cc
+++ b/aos/linux_code/logging/log_displayer.cc
@@ -154,7 +154,7 @@
         }
         break;
       case 'p':
-        if (!::aos::util::StringToInteger(::std::string(optarg), &source_pid)) {
+        if (!::aos::util::StringToNumber(::std::string(optarg), &source_pid)) {
           fprintf(stderr, "ERROR: -p expects a number, not '%s'.\n", optarg);
           exit(EXIT_FAILURE);
         }
@@ -177,7 +177,7 @@
         skip_to_end = true;
         break;
       case 'm':
-        if (!::aos::util::StringToInteger(::std::string(optarg), &display_max)) {
+        if (!::aos::util::StringToNumber(::std::string(optarg), &display_max)) {
           fprintf(stderr, "ERROR: -m expects a number, not '%s'.\n", optarg);
           exit(EXIT_FAILURE);
         }