Description of proposed changes

See #3408 for the initial problem. Here is what goes wrong:

1. We first read the container header from a file. The file is a memory reference to a matrix. This returns a grid structure with a header.
2. Then grdgradient is called with the same file to make an intensity grid. This all works.
3. Later, we wish to read the data from the matrix source. However, despite me implementing more meaningful memory file names like @gmtapi@-S-I-G-M-G-N-000000, the old code has not been updated: It loops over the registered objects and finds the first one that matches the specs (image, input, etc). This turns out to be the grid header from (1) above instead of item 0. So we go down the rabbit hole and do strange things.

Time to implement a new scheme that extracts the information directly from the filename.

Closes #3408.