Add more metadata to grid file

[dschwoerer]

Should we include the version of zoidberg in the grid files?

[ZedThree]

I've spent a fair bit of time this week reading various metadata conventions. My opinion is that we should add the following as global attributes to every file we create:

• title: something like BOUT++ grid file
• software_name: zoidberg
• software_version: full version
• date_created: ISO 8601 timestamp of this run
• id: a UUID for this run

And possibly the following:

• netcdf_version: This is nc_inq_libvers() or netCDF4.getlibversion()
• Conventions: URL of units/normalisations documentation (not sure this actually exists, but it should!)
• references: DOI of zoidberg/BOUT++
• comment: human-friendly description of this run, e.g. W7-X shot #1234

All variables in the file may have the following attributes:

• units: SI units
• long_name: human-friendly description

I've based this mostly on the netCDF and CF metadata conventions.

[ZedThree]

Partly done in #13