Add bpm_calib directory with scripts and documentation for BPM calibration (#475)

* Updates for new THcRaster class

* Add bpm_calib directory with scripts and information on BPM calibration
(from HARP scans).

* Update ReadMe

* Update gbeam.param and add diagram off Hall C raster algorithm for
documentation purposes.

* Update diagram

* Update documentation, add output of PARAM file lines to calibration
script.