Upated readme
Merge branch 'master' of https://github.com/willo12/spacegrids

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Added strict_v option to field method __sub__. Previously it was only in add (forgot sub). Also added another check to add and sub to see if self and other grid are more strictly the same. If not, a warning is issued (sameness of coord points suffices). Added some comments to field class. Introduced a same method to coord and ax, and related methods, also extending to gr objects. This is more strict that == but of course less strict than is. Not sure how in works at the moment (related to ==? or pointer identical?), as in:  if e in gr: