Better handling of tiled grids

**Description of the desired feature**

While the tiled remote grids allow fast response for smaller regions, if you actually want to use the full global grid at that resolution, you are downloading all the tiles and then stitching them together. The subsequent times the tiles are there but you still end up stitching. This is particularly wasteful if using a global tiled dataset in an animation. Here are some suggested changes to **grdblend**:

1. When requesting a global tiled grid with **-Rd** or **-Rg** (explicitly or implicitly), first check if a single grid exists before doing the tiling. If the global file exists, use it.
2. First time assembling a full global grid, write a copy of the assembled grid (with **-Rd**) to the corresponding server directory so it can be used as is next time.
3. Delete the tile subdirectory after creating the single global grid.

While at it, we should add a special "stitch" mode to **grdblend** since right now there is some overhead in setting up and computing weights for blending which are not used.