Some support for data from BOUT++ v3.x

Add several workarounds to allow data from simulations run using BOUT++ v3.x to be loaded, and allow (some) xbout methods to work.

Workarounds:

get topology information - jyseps1_1, etc., from the grid file
add a direction_y attribute to Field3D variables, which is always "Aligned" for BOUT-3 data.
add a cell_location = "CELL_CENTRE" attribute to all fields. The cell_location was not recorded by BOUT++ in v3. Also, staggered grid support in BOUT++ was only partial so treating all variables as if they were CELL_CENTRE may be consistent with BOUT-3 (e.g. when shifting between aligned and non-aligned grids, always using zShift from CELL_CENTRE is consistent with BOUT-3).
set nz = MZ-1 in the metadata.
pass data_vars = "different" to work around hist_hi and tt (which are expected by xbout) not being present - effect is to concatenate any variables that are different in different files (this may be slightly slower).
in _read_splitting(), calculate nx and ny from MXSUB, NXPE, MXG, MYSUB, and NYPE, as they were not written to output files by BOUT-3.
skip handling for squashed data files that include upper target boundary cells - it is not (conveniently...) possible to test whether the output contains these cells, as the output files did not contain jyseps2_1 or jyseps1_2, but anyway bout-squashoutput did not save those cells at the time, so skipping the handling should not cause a problem. If the upper target cells were somehow present, an error (although with an unhelpful message) would be raised (I think...).

boutproject / xBOUT