GridSpacing: Per-Component UnitSI & UnitDimension

[ax3l]

The gridSpacing should get a unitSI and a unitDimension for each of its components.

This allows to describe grid such as x, ux or x,y,t (although the "time" definition per iteration would then be "for the first row").

[RemiLehe]

For gridUnitSI this seems quite simple: we just enter an array of floats (one element per dimension) instead of a single float.
For gridDimension, it seems that the most natural thing would be to have a 2d array of dimension Nx7, where N is the number of dimensions. However, I am not sure that HDF5 (and others) support 2d arrays as attributes. Therefore, I guess that the best choice would be a flattened 2d array. @ax3l Any opinion?

[ax3l]

Sorry I missed the question. Yes, HDF5 does support 2D arrays as attributes, ADIOS afaik is limited to 1D arrays right now and as you found out: h5py does not expose that feature. So flattened to the same index order as the data (from fast varying index to slowest) is fine for me as well.