only accept lower case to be consistent with the rest of the input

src/utils.cpp

@@ -372,7 +372,6 @@ int utils::logical(const char *file, int line, const char *str, bool do_abort, L
   // convert to ascii and lowercase
   std::string buf(str);
   if (has_utf8(buf)) buf = utf8_subst(buf);
-  std::transform(buf.begin(), buf.end(), buf.begin(), tolower);
 
   int rv = 0;
   if ((buf == "yes") || (buf == "on") || (buf == "true") || (buf == "1")) {

src/utils.h

@@ -159,8 +159,8 @@ namespace utils {
                                        LAMMPS *lmp);
 
   /*! Convert a string to a boolean while checking whether it is a valid boolean term.
-   *  Valid terms are 'yes', 'no', 'true', 'false', 'on', 'off', '1', '0'. Capitalization
-   *  will be ignored (thus 'yEs' or 'nO' are valid, 'yeah' or 'nope' are not).
+   *  Valid terms are 'yes', 'no', 'true', 'false', 'on', 'off', and '1', '0'. Only
+   *  lower case is accepted.
    *
    *  \param file     name of source file for error message
    *  \param line     line number in source file for error message