GR API: Remove length attributes from Python methods.

The length attributes in the following methods have been removed
because the length information for sequences can be determined
at runtime in Python. Additional checks have been implemented which
will raise an AttributeError if supplied sequences have wrong lengths.

def polyline(x, y):
def polymarker(x, y):
def fillarea(x, y):
def spline(px, py, m, method):
def gridit(xd, yd, zd, nx, ny):
def verrorbars(px, py, e1, e2):
def herrorbars(px, py, e1, e2):
def polyline3d(px, py, pz):
def surface(px, py, pz, option):
def contour(px, py, h, pz, major_h):
release v0.5.0