New r8th routing file, pylint and py3 checking, fill value needed for pio2

Add in 8th degree routine file. Run pylint and check for py3 compatability.
There's explict setting of fill type. And also explicit use of shr_kind_r4 for
kind rather than real(4), which is a better mechanism. Also ncd_getiodesc
will read in PIO_DOUBLE for input xtype= PIO_DOUBLE or PIO_REAL. Most of this
is direct from jedwards4b (other than r8th addition).