[Eg6_analysis] RTPC alignment note

[Nathan Baltzell]

Dear EG6ers,

Here is the note on EG6 RTPC alignment calibration:

https://userweb.jlab.org/~baltzell/EG6/RTPC/tpcalign_v2.pdf

It has a modification from the version I posted in last week's
meeting, because I realized a study exists from eg1b of the
effect on CLAS tracking in solenoid field due to wrong beam
position.  So I applied their corrections to eg6 1.206 GeV
elastic kinematics and updated the note accordingly in
Section III.  I'll submit it as a CLAS-NOTE next week unless
you disagree.

Gain calibration note coming soon.

Regards,
Nathan