overly precise cell dimensions test creates issues with non square cell sizes

[craigmillernz]

The test for nonsquare cell sizes could be relaxed so that this error does not occur. Perhaps some rounding of grid dimenions prior to this test would be good.

fft = gx.grid_fft.GridFFT(grd)
File "C:\Users\xxxxx\Anaconda3\lib\site-packages\geosoft\gxpy\grid_fft.py", line 223, in __init__
  raise GridFFTException(_t('Cell size must be square'))

My grid cell sizes only vary in the 13th? decimal place of a meter.

grd.dx
Out[9]: 99.658858107433**62**

grd.dy
Out[10]: 99.658858107433**59**

proposed solution line 222/223 of grid_fft.py

if np.round(grid.dx, 10) != np.round(grid.dy):
            raise GridFFTException(_t('Cell size must be square'))