Can't compile my library because "String does not name a type"
Your syntax is wrong in myESP.cpp. Change this:

myESP::String macToStr(const uint8_t* mac) {
}

myESP::String doGet(String data, String sensor, int duration) {

}


to this:

String myESP::macToStr(const uint8_t* mac) {
}

String myESP::doGet(String data, String sensor, int duration) {

}


The return type is String and the function implementation should be prefixed with the class name myESP::. Obviously, you also need to fill in the body of the implementation or you'll get more errors because you don't actually return anything.