[Eg6_analysis] 1gev beam offset and RTPC

[Nathan Baltzell]

Dear EG6ers,

Followup to meeting:
I tried to make the same plot Stepan showed for beam offset,
but for the 1GeV data used to calibrate RTPC.  Then I looked
at the effect on RTPC reconstruction:

http://clasweb.jlab.org/rungroups/lowq/wiki/index.php/Beam_Offset

Result: ~mm beam offset is important for helix fit.

Note, RTPC misalignment should have same effect here as beam offset.
To try to measure it, could look at elastic track kinematic vs phi.
Or maybe z_RTPC-z_CLAS vs phi after correcting z_CLAS would be  
better/easier.

-Nathan