gmtconvert is not able to return time converted input from wrappers

```
using GMT

result = sample1d([0 0; 4 1], inc=0.5);
gmtconvert(result, f="i0t/o0T", par=(TIME_EPOCH="1969-07-21T02:56:00", TIME_UNIT=:d))
```

Returns the same as input (telling it to save to file, works). The reason seems convoluted as the workflow follows quite different paths for return as memory address (the wrappers output) or when saving to file.