vdot on subspaces

Hi Martin,

I just recognised that ift.Field.vdot does only work on 'whole' fields and does not yet support .vdot on subspaces.

In [1]: import nifty2go as ift

In [2]: x1 = ift.RGSpace(200)

In [3]: x2 = ift.RGSpace(150)

In [4]: m = ift.Field((x1,x2), val=.5)

In [5]: m.vdot(m, spaces=1)
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
<ipython-input-5-5c9f18522742> in <module>()
----> 1 m.vdot(m, spaces=1)

/Users/danielpumpe/CloudStation/dpumpe/construction/NIFTy_2go/lib/python2.7/site-packages/nifty2go-3.9.0-py2.7.egg/nifty2go/field.pyc in vdot(self, x, spaces)
    339             return fct*dobj.vdot(y.val, x.val)
    340 
--> 341         raise NotImplementedError("special case for vdot not yet implemented")
    342         active_axes = []
    343         for i in spaces:

NotImplementedError: special case for vdot not yet implemented

What it be possible to provide this functionality?