upgrade SCORPIO interface to support integer*8 "dims" arrays

For NE>=6144 (500m), the description of the atmosphere restart arrays need to be upgraded to integer*8

In addition, the SCORPIO pio_initdecomp() function will need to be upgraded to support "dims" array of size integer*8

We wont be running at this resolution anytime soon, so this is more for future reference.

This is an updated version of issue #2698
Also documented https://github.com/E3SM-Project/scorpio/issues/156

The size of a horizontal arrays in HOMME:
GLL restart arrays are of size 16\*6\*nelem^2, which exceed 2^31 at NE=4730 (EAM only)
GLL "unique pts/ncols" arrays, size 9\*6\*nelem^2 + 2, which exceed 2^31 at NE=6307 (EAM & EAMXX)