[params] Runtime error when casting string param to const char*

[galexv]

The following code:

#include <alps/params.hpp>
using alps::params;
using std::string;

///Some function that requires a C string
void consume_c_str(const char*) {}

int main(int argc, char**argv) {
  params p(argc,argv);
  p.define<string>("str", "default", "A string param");
  // Attempt to pass a parameter as a C-string
  consume_c_str(p["str"]); // Throws!
  // Attempt to obtain a C string:
  const char* my_c_str=p["str"]; // Throws!
}

throws type_mismatch exception about illegal attempted conversion from string to char*.
This looks like a bug: either it should perform the conversion and return a const char* pointer to the underlying string (whether it's a good idea is a different question), or it should fail at compile time, as it does with other unsupported types.