Fix shared plane index in BEAMnrc block CM (#248)
Fix shared plane index in BEAMnrc block CM (#248)

Fix plane index when looking for shared planes, when the shared edge is 
the last edge of a subregion. Previously, trying to find an intersection 
could lead to an infinite loop. Also correct a typo and remove an 
unnecessary distance check.
ftessier/EGSnrc,ftessier/EGSnrc,mchamberland/EGSnrc,crcrewso/EGSnrc,marenaud/EGSnrc,mchamberland/EGSnrc,nrc-cnrc/EGSnrc,nrc-cnrc/EGSnrc,ftessier/EGSnrc,crcrewso/EGSnrc,mchamberland/EGSnrc,marenaud/EGSnrc,ftessier/EGSnrc,mchamberland/EGSnrc,mchamberland/EGSnrc,MartinMartinov/EGSnrc,MartinMartinov/EGSnrc,MartinMartinov/EGSnrc,marenaud/EGSnrc,crcrewso/EGSnrc,mchamberland/EGSnrc,MartinMartinov/EGSnrc,nrc-cnrc/EGSnrc,nrc-cnrc/EGSnrc,crcrewso/EGSnrc,marenaud/EGSnrc,crcrewso/EGSnrc,nrc-cnrc/EGSnrc,marenaud/EGSnrc,marenaud/EGSnrc,ftessier/EGSnrc,ftessier/EGSnrc,MartinMartinov/EGSnrc