remove __len__ method

the reason is that it confuses numpy's ufunc which think that a chebfun is an iterable
test that ufuncs return chebfun objects
cleanup chebfun comparisons in tests
add slicing

now possible to extract one component of a chebfun
unified treatment of operator overloading
fix scalar-vector multiplication
fix equality test
minor change in tests
cleanup and thorough testing of chebfun initialisation function
add docs/comments in chebfun class
move basis to a class method Chebfun.basis
fix identity call in readme
move size in attribute section
private variable in for loop outside class