RTCM: Add option to force NTRIP rx clock error to zero

This is done by estimating the clock error by lsq then subtracting
that from the PRs before transmission.  If enabled, a second lsq
run is done after correction to ensure that it has in fact worked
(mainly a debugging thing)
positionEstimate: Move rx clock estimation in from RTCM code

This will allow its use for the local corrections as well
RTCMv2: Fix an occasional key error for sats with partial ephemeris
Local: Solve for clock err using reference pos if available

Previously we did a full 4D solve even if the spacial dimensions were known.
This now means we're only optimizing over one variable and should get better accuracy