Implement multiple species restart, and drift electrons

Btinf is removed and the infinite field test is now whether the
angle of gyro rotation exceeds 1, for each species.
Particle writing and reading is reorganized to include all
species. Old files ought still to be readable.
Got restarting working with electrons.
Implemented lengthened file extension format for high step counts.
Add checksp for multiple species restart checking

Update uniformelec.cks. Remove spurious test writes in partwriteread.