try convert string to numeric type

[Peter]

There is the convert function to convert a string to numeric type and there is the is function to check if a string can be converted to a numeric type.

Smetimes you don't know if your string can be converted, but if it can you want to convert it. Rather than writing something like

int x=0;
if (is<int>(str))
  x = convert<int>(str);

it would be better to write something like

int x=0;
try_convert<int>(str, x);

I think the function already is implemented as a private function. Function try_convert should perhaps return bool` as an indication whether str was converted or not.

[Peter]

(In [3597]) closes #828, function convert_try