Empty loops cause braidlab to crash

[mbudisic]

At this point, empty coordinate vector [] ( 0x0 double ) and zeros(1,0) (1x0 double) cause different behavior for length functions. In particular, this leads to the segfault of MEX intaxis function, while forcing Matlab's implementation of intaxis causes just regular Matlab error about mismatched indices to be thrown.

For example:

>> C = zeros(1,0)
>> L = braidlab.loop(C);
>> L.intaxis

[mbudisic]

For now, I am going to disallow passing an empty coordinate vector to braidlab.loop constructor.

[jeanluct]

See also issue #128.