Incorrect Field Preallocaiton

There seems to be a typo in GrapsField "read" function:

self.field is preallocated based on X, Y, and filed components dimensions.

`self.field = numpy.zeros(shape=(self.grid_n_x, self.grid_n_y, self.field_components), dtype=complex)`

When reading a grid file, the loop iterates, and populates ``fields", based on Y then X dimensions (i.e., j then i), hence causing an error. 