Replace some of TAxisConfig's ctors with concrete axis conversion ops.
Replace some of TAxisConfig's ctors with concrete axis conversion ops.

TAxisConfig construction was ambiguous due to too many constructor
overloads. Instead of constructing a TAxisConfig from a TAxisEquidistant etc,
allow the latter to convert to the former. This also gets rid of TAxisConfig's
init_list ctors.

gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root,gganis/root