remove hard coded font sizes from plot_smith_grid in vna.py
remove hard coded font sizes from plot_smith_grid in vna.py

pavel-demin/red-pitaya-notes,fbalakirev/red-pitaya-notes,fbalakirev/red-pitaya-notes,pavel-demin/red-pitaya-notes,pavel-demin/red-pitaya-notes,fbalakirev/red-pitaya-notes,fbalakirev/red-pitaya-notes,pavel-demin/red-pitaya-notes,fbalakirev/red-pitaya-notes,pavel-demin/red-pitaya-notes,pavel-demin/red-pitaya-notes,fbalakirev/red-pitaya-notes,fbalakirev/red-pitaya-notes,pavel-demin/red-pitaya-notes,pavel-demin/red-pitaya-notes