[GRASSLIST:3317] 5.7 coor format changed

[Radim Blazek]

I have extended native vector format in 5.7. 
The vectors written in new format cannot be read by old verion of 5.7
but old vectors may be read by new version of 5.7.
The change will be done in 2 steps. Today I have submited the code, 
which can read both old and new version of vectors. 
The code for writing is also present, but compiled only if GV_FORMAT_51 is defined.
I'll enable the code for writing in next weeks. 
You should update your GRASS during this period, if you share GRASS data with other 
people.

Radim