fixing issue Issue174 related to implicit functions

[benaliabderrahmane]

#implicit function:

the code was tested with the following examples:

fimplicit(@(x,y) x.^2 - y.^2 - 1)
fig2plotly(gcf,'offline',false);

##results:

f = @(x,y) x.^2 + y.^2 - 3;
fimplicit(f,[-3 0 -2 2])
fig2plotly(gcf,'offline',false);

f1 = @(x,y) x.^2 + y.^2 - 1;
fimplicit(f1,':r')

hold on
f2 = @(x,y) x.^2 + y.^2 - 2;
fimplicit(f2,'--g','LineWidth',2)
hold off
fig2plotly(gcf,'offline',false);