Sampcrit class + example script for EHVI criterion

[jbect]

The EHVI criterion is available through the stk_sampcrit_ehvi_eval function since STK 2.4.0.

The corresponding sampcrit class is missing. Also, an example script would be useful.

(original ticket on Sourceforge: https://sourceforge.net/p/kriging/tickets/83/)

(meta-issue about sampcrit classes: https://github.com/stk-kriging/stk/issues/10)