grdinfo on tiled files may be overkill

For example, this one already take a lot of time and uses an extra 1 GB of mem
```

gmt grdinfo @earth_relief_01m
GMTAPI@-S-O-G-G-G-Y-000002: Title:
GMTAPI@-S-O-G-G-G-Y-000002: Command: grdblend @earth_relief_01m_p/ -R-180/180/-90/90 -I01m -rp -G@GMTAPI@-S-O-G-G-G-Y-000002 -Co+n
```
The problem is obviously because it has to assemble all the tiles to compute the info. A possible solution would be to store this info together with the files and add another cleverness to the reader to consult that file first if no -R is provided.