Material ids are not output correctly when using field_dump

[jjaraalm]

Trying to output material_ids is broken

vpic/src/vpic/dump.cc

Lines 643 to 644 in cd46b8a

	const uint32_t * fref = reinterpret_cast<uint32_t *>(&f(i,j,k));
	fileIO.write(&fref[varlist[v]], 1);

and

vpic/src/vpic/dump.cc

Lines 659 to 660 in cd46b8a

	const uint32_t * fref = reinterpret_cast<uint32_t *>(&f(ioff,joff,koff));
	fileIO.write(&fref[varlist[v]], 1);

assume that all fields of a field_t are 4 bytes wide, but material_id is only 2 bytes. The resulting output is not as expected.