Incorrect unit (dpi instead of dpm) used for resolution (xres, yres)

The incorrect unit is used both in the C++ codebase, as well as in the .py file. In both cases "dots per inch" are used, while the specification states that the unit is "per meter" (page 104 in https://github.com/agl/jbig2enc/blob/ea050190/fcd14492.pdf).