Add gain and range setting

[caternuson]

Re this thread:
https://forums.adafruit.com/viewtopic.php?p=938230#p938230

Add ability to get/set the various gain and range settings.

[aaronwmorris]

I have submitted PR #21 which includes the gain and high range setting.

[jposada202020]

@FoamyGuy I think this can be closed now :) after PR 21

[aaronwmorris]

I agree, everything is working as expected.

[caternuson]

Closing. Should be resolved by #21.