MINOR: add rcond parameter to use future precise numpy default

[sappelhoff]

np.linalg.lstsq currently elicits a Future Warning:

FutureWarning: `rcond` parameter will change to the default of machine precision times
 ``max(M, N)`` where M and N are the input matrix dimensions.
To use the future default and silence this warning we 
advise to pass `rcond=None`, to keep using the old, explicitly pass `rcond=-1`.
  mapping = np.linalg.lstsq(X, y)[0]

In this PR, I set the rcond parameter to rcond=None... to adopt the future Numpy default for this line:

xdf/Python/xdf.py

Line 540 in 4e02de7

mapping = np.linalg.lstsq(X, y)[0]

If we want to keep the old behavior, we should set rcond=-1. However, using the future default is probably better. See also here.

[cboulay]

Thanks, and sorry for taking so long to get to this.