Small improvement to lengthscale handling

[bwengals]

Also changed classes to call super instead of SuperClass.__init__(self, ...)

Allows user to set multiple lengthscale parameters individually like this:

with model:
    l1 = pm.HalfCauchy("l1", beta=5)
    l2 = pm.HalfCauchy("l2", beta=5)
    cov = pm.gp.cov.ExpQuad(2, [l1, l2])

[fonnesbeck]

LGTM

[ferrine]

Nice:)