Create BoutOptionsFile with grid if available

[mikekryjak]

Loading a BOUT++ dataset will automatically create a BoutOptionsFile, which at the moment will want to read the grid file to evaluate input file expressions with .recalculate_xyz(). However, it can't easily find the grid file on its own - it will assume it's in the working directory, which is nearly always incorrect in my case. It will then try to find nx and ny in the input file, which is always incorrect in my case.

This PR constructs a BoutOptionsFile using the already known gridfilepath if it's available, which it always is in my use case. If it's unavailable, it falls back on the previous behaviour.

This is connected to boutproject/boutdata#145. I'm trying to fix all the warnings I get when loading datasets.

[dschwoerer]

BoutOptionsFile expects a filename for gridfilepath, but it may also be just the folder in which the grid from the options file can be found, see

xBOUT/xbout/load.py

Line 373 in fcf70c1

if _is_dir(gridfilepath):

[mikekryjak]

@dschwoerer nice find, I actually didn't know this. If it's a directory, we can read the options file without the grid to get the name, and then read it with the grid.

On a separate note, isn't this wrong? What is options:grid? I've only ever seen options:mesh:file!

[dschwoerer]

Legacy stuff:
https://github.com/boutproject/BOUT-dev/blob/next/src/mesh/mesh.cxx#L54

[mikekryjak]

@dschwoerer done, take a look

[dschwoerer]

We no might warn twice about that the simulation was run without grid, but who would run simulations without grids 😇