[Sane-analysis] Approximate correction for target field deflection

[O. A. Rondon]

Hi collaborators,

I have posted a wiki section on a simple way of making an approximate
correction for the target field deflection
https://hallcweb.jlab.org/experiments/sane/wiki/index.php/Simple_Correction_for_the_Target_Field_Deflection

It is mostly for use with GEANT output or for quick tests of data
quantities, such as angles, that may need correction for the deflection,
since it can be used in paw.

It is NOT a substitute for the reconstruction of trajectories based on
the Neural Networks, due to its approximate nature (uniform field is
assumed).

The correction is implemented in subroutine reconstruct.f called by the
simple GEANT reconstruction code pmc, which reads ntuples generated with
a1p_mc (old G. Warren - GW version; it needs modification to read in the
new SANE GEANT version). The quad fit coefficients to Bdl(theta_s) are
given in both reconstruct.f and on the wiki.

The assumption throughout is that the coordinate system for particle
trajectories in use is that described in the comments at the beginning
of subroutine generate_event in gen_evt.f, part of the a1p_mc montecarlo
(SANE or GW), and the detector geometry systems follow the standard
GEANT coordinates described in the GEANT3 manual (online at:
http://wwwasdoc.web.cern.ch/wwwasdoc/geant_html3/node109.html#SECTION041000000000000000000000

Cheers,

Oscar