Replaced stringstream stod and stoi implementation

Replaced stringstream stod and stoi implementation with a wrapper around strtod and strtol. It checks for the same conditions as stod and stoi, but provides the same interface as strtod and strtol.

Using that interface, I was also able to remove all calls to string::substr within datatable.cpp. This could dramatically speed up loading as it avoids copying parts of the loaded string over and over again.

Instead, it just uses a bit of pointers to move ahead on the original string, so no string copying is done.