Added powerSaveMode

[PaulZC]

Hi Nathan (@nseidle),
I think this should be my last PR for a while!
I have added powerSaveMode which uses the UBX-CFG-RXM message to put the GNSS into power save mode. You can put the receiver back into continuous mode by calling powerSaveMode(false).
There is, of course, an example too.
I've tested it on the ZOE-M8Q and the current drops from 25-28mA during acquisition to approx. 9mA when low power mode kicks in. (That's with a passive antenna. Using an active antenna will add 10's of mA to the total draw.)
Enjoy!
Merry Christmas,
Paul

[nseidle]

I'm generally ready to merge. Two topics to discuss:

• The power save feature works well for my SAM-M8Q but fails on the ZED-F9P as it doesn't have support for the -CFG-RXM command (it uses UBX-RXM instead). Is there a better way to test and warn the user that a given command isn't supported? Best case is a Serial.print("This command is not supported on this hardware"). Next best thing is to notify user when a command causes a UBX-ACK fail (as was the case with ZED-F9P and NAV-CFG-RXM).

• I'm making a bit of a mess of this PR. I'm attempting to remove/reject the files that are really parts of other PRs. It seems I'm supposed to ask you to fix the PR.

Please remove the dynamic model and geofence bits in this PR. If that's too much work, I could copy/paste your changes locally and apply the bits I want, then close this PR but you get none of the glory ;). Let me know if you want to attempt to fix this PR or if I should copy/paste.

In the future, please branch from master rather than your own branches. And I am far from a git guru so if you know of a way that I can merge just the power save bits, let me know.

[PaulZC]

Closing this for now until I understand the root cause of the timeout failures from #52 (which also apply to this function). I'll re-submit from a fresh fork.
Bye for now!
Paul