Fix nrc-cnrc#425: irold not updated in electron step loop

Assign the current particle region, ir(np), to the old region index,
irold, within the ustep loop in electron transport. Previously, irold
was not updated, so the irnew ~= irold condition added to fix magnetic
field transport in vacuum caused zero-size steps to skip the region
change macro. Remove the now redundant irold = ir(np) assignment in the
non-zero step size block.

Fix nrc-cnrc#260: rest mass not initialized in C interface

Fix the rest mass initialization for EGSnrc applications that depend on
the C interface. The bug fix in the parent commit moved the declaration
of the rest mass inside the COMIN/USEFUL/ macro. However this in turn
breaks C and C++ EGSnrc applications because the C interface relies on
its own declaration of the COMIN/USEFUL/ macro, which also has to be
updated with the rest mass data statement.

Merge pull request nrc-cnrc#33 from nrc-cnrc/develop

Merging develop branch into master

Merge pull request nrc-cnrc#12 from nrc-cnrc/develop

Merging develop branch into master