Initializes a grid of axes with adjustable gaps and margins and returns their handles.
Credit: Pekka Kumpulainen (2010) @tut.fi Tampere University of Technology / Automation Science and Engineering
rows (integer): Number of axes in hight (vertical direction).
columns (integer): Number of axes in width (horizontal direction).
gap (double): Gaps between the axes in normalized units (0...1) or [gap_h gap_w] for different gaps in height and width.