Remove use of non-existent EvalDir option.

additionally allow all options to be converted to string.
Without this, restoring of the options (multi_think.cpp:117) can't work.

fixes https://github.com/nodchip/Stockfish/issues/128

Now gensfen/learn pass with debug=yes in CI

src/ucioption.cpp

@@ -165,7 +165,7 @@ Option::operator double() const {
 }
 
 Option::operator std::string() const {
-  assert(type == "string");
+  assert(type == "check" || type == "spin" || type == "combo" || type == "button" || type == "string");
   return currentValue;
 }