fix indexing for fgmax point_style==4

[rjleveque]

fgmax_tools.read_output has an indexing parameter introduced in #561 that was not working properly for point_style==4 (fgmax points specified by a topofile mask).

This should now allow setting indexing = 'ij' or 'xy' to specify how the resulting array attributes X, Y, h, etc are indexed, and so that the 1D arraysfgmax.x and fgmax.y are also set properly.

Also fgmax.indexing is set by read_output as a convenience for the user.

Also cleaned up a couple other related things and added some comments to clarify the code.