Handle duplicated points in stk_predict (noisy case)

[jbect]

We should automatically detect and group duplicates in noisy GP regression, since it allows to reduce the dimension of the kriging system.

See also iago/rep directory in the master branch & ongoing work concerning the stk_iodata class (here: #3).

(original ticket on Sourceforge: https://sourceforge.net/p/kriging/tickets/85/)