'set_gain' returns the last value of 'read'

[Nagasaki45]

When two load cells are connected to one HX711 it is a trivial use case to read both channels in a loop. Currently it looks something like this:

void loop() {
  scale.set_gain(64);
  long loadA = scale.read();
  scale.set_gain(32);
  long loadB = scale.read();
  // Do something with readings...
}

In such scenario, under the hood there are 4 calls to read which slow down everything significantly.

With this change, this use case can be solved efficiently with:

void loop() {
  long loadB = scale.set_gain(32);
  long loadA = scale.set_gain(64);
  // Do something with readings...
}

I know that it an awefull interface to work with, as the set_scale method already does too many things. However, I couldn't find a better yet simple way to both read and update efficiently. If you have a better solution I would like to hear!

Note that this change shouldn't break existing code (AFAIK, I'm not a C++ dev).