[Eg6_analysis] rtpc alignment

[Nathan Baltzell]

Dear All,

Continuing the discussion from today's meeting regarding
the possible effect of misalignment on momentum, I looked
into the reconstruction code and made some estimates from
the magnetic field map.

For details:
http://clasweb.jlab.org/rungroups/lowq/wiki/index.php/Alignment

To summarize:
The effect should be a few-percent error on momenterm per cm
of misalignment due to picking the incorrect magnetic field
 from the map.  It's dependent on polar angle, but at elastic
kinematics we would need much greater than 1cm of misalignment
for this effect to wholly account for the L/R momentum
discrepancy we see in the data.

-Nathan