Positions along a path

There are several methods to access certain positions at a path. At first there
are `atbegin` and `atend` methods, which return a coordinate tuple. ...

! In this example, we have defined a small helper function `mark` to which we
can pass the return value of the `at...` methods. We do this by transforming
the sequence in to positional argument of the function call. This Python
language feature is available by the `*` syntax in the call.

! The coordinates returned by the `at...` methods are PyX lengths in the
unscaleable true units similar to the return value of the `arclen` method.

For the left path `p1`, we also show how to use the `at` method,
which can be used to get the coordinates of a certain point of the path
depending on the arc length along the path.

A different use of the `at` function is shown for the right path `p2`: It is
also possible to pass parametrization instances of the path to the `at` method.
In the shown case we first fetch such parametrization instances for the
beginning and the end of the path by the `begin` and `end` methods. Note that
the `atbegin` method is equal to call `at` with the result value of the `begin`
method (except for optimizations: atbegin is faster than the two calls).
Similar `atend` could be rewritten using the result value of `end`. As shown in
the example you can use the parametrization instances to add and substract arc
lengths from selected point.

! The `at` method of a path instance can not only handle a single value, in
which case it returns a single result tuple. You can also pass a list to the
method and the return value will contain a list of coordinate tuples. You
should bear in mind that passing a list is much more efficient than multiple
calls of the method performing single conversions at each time.