deco.arrow: introduce relarclenpos and friends

deco.arrow: introduce relarclenpos and friends

The pos argument of deco.arrow() maps the range [0,1] to the range of
available positions such that the arrow head can be constructed from the
path without extrapolation. This is useful especially for begin and end
heads, but different from the positioning of other decorators such as
deco.text().

Introduce additional position parameters relarclenpos, arclenfrombegin
and arclendfromend which work exactly like for other decorators. They
all default to None, which means that the pos argument is used and the
behaviour of deco.arrow() is unchanged unless you specify a value for
one of the new arguments.

Also, introduce a new center argument which specifies the center of the
arrow head (multiples of its size). It defaults to the center of mass of
a solid triangular arrow head but is used only when any of the new
position parameters is specified.