Smooth transition of elements from one size to another based on contour levels

[BahramKhazaei-NOAA]

One simple way to develop a new approach in OCSMesh for smooth transition between two element/cell size values is to consider multiple contour lines between the two sizes. That can be implemented by defining size at different contour levels as follows:

# level 1:
hfun.add_contour(target_size=750,  level=0,  expansion_rate=0.025)
hfun.add_constant_value(value=750, lower_bound=0)

# level 2:
hfun.add_contour(target_size=2000, level=-1500, expansion_rate=0.01)
hfun.add_constant_value(value=2000, lower_bound=-1500,  upper_bound=-2500)

# level 3:
...

Another way is to just define many levels in the example shown below and define contour levels through a loop:

# Size function criteria for 25 contour levels:
levels   = np.linspace( 0, -7500, num=25)
exp_rate = np.linspace(0.05,  0.01, num=25)
tgt_size = np.linspace(1000,  7500, num=25)

# Define 24 contour levels based on the criteria above:
for il in range (0, len(levels)-1):
    hfun.add_contour(target_size=tgt_size[il], level=levels[il], expansion_rate=exp_rate[il])
    hfun.add_constant_value(value=tgt_size[il], lower_bound=levels[il])

# Define one last level (contour level 25):
hfun.add_contour(target_size=tgt_size[-1],  level=levels[-1],  expansion_rate=exp_rate[-1])
hfun.add_constant_value(value=tgt_size[-1], lower_bound=levels[-2], upper_bound=levels[-1])