Urls don't work for everybody

[mfornet]

Some functions were tested on problem not accessible to everyone.

• tangent function in circle.cpp
• minimal_circle.cpp
• Semiplane intersection tested on problem from Gleb camp and it is not accesible.

[gilcu3]

for the third problem, I think it could be tested here (apparently same problem)

for the first two algorithms, although inside a cf group, I think that group is open to anyone...