Introduce Overload.

[31m059]

This is my first time opening a PR, so I apologize if there are errors.

There are too many people to thank since I submitted my first test just over a month ago. Thank you all for the warm welcome and here's to more green patches!

In particular, I would like to thank:
@crossbr, whose comment in a FishCooking thread first inspired me to consider the overloading of pieces other than queens,
@snicolet, whose queen overload tests inspired this one and served as the base of my first overload attempts,
@protonspring, whose connectivity tests inspired this one and who provided much of the feedback needed to take this from red to green,
@vondele, who kindly corrected me when I submitted a bad LTC test, and
@Rocky640, who has helped me over and over again in the past month.

Thank you all!

This patch applies a S(10, 5) bonus for every square that is:

• Occupied by an enemy piece which is not a pawn
• Attacked exactly once by our pieces
• Defended exactly once by enemy pieces

The idea is that these pieces must be defended. Their defenders have dramatically limited mobility, and they are vulnerable to our future attack.

As with connectivity, there are probably many more tests to be run in this area. In particular:

• I believe @snicolet's queen overload tests have demonstrated a potential need for a queen overload bonus above and beyond this one; however, the conditions for "overload" in this patch are different (excluding pieces we attack twice). My next test after this is (hopefully) merged will be to intersect the Bitboard I define here with the enemy's queen attacks and attempt to give additional bonus.
• Perhaps we should exclude pieces attacked by pawns--can pawns really be overloaded? Should they have the same weight, or less? This didn't work with a previous version, but it could work with this one.
• More generally, different pieces may need more or less bonus. We could change bonuses based on what type of enemy piece is being overloaded, what type of friendly piece is attacking, and/or what type of piece is being defended by the overloaded piece and attacked by us, or any intersection of these three. For example, here attacked/defended pawns are excluded, but they're not totally worthless targets, and could be added again with a smaller bonus.
• This list is by no means exhaustive.

STC:
LLR: 2.96 (-2.94,2.94) [0.00,5.00]
Total: 17439 W: 3599 L: 3390 D: 10450
http://tests.stockfishchess.org/tests/view/5ac78a2e0ebc59435923735e

LTC:
LLR: 2.95 (-2.94,2.94) [0.00,5.00]
Total: 43304 W: 6533 L: 6256 D: 30515
http://tests.stockfishchess.org/tests/view/5ac7a1d80ebc59435923736f

Bench: 5248871

[snicolet]

@31m059

To get the travis Continuous Intregation happy in a pull request, the last commit on the branch must have bench too. In the present case you can make a dummy commit for this by removing the digit at the end of line 612 (one line comments do not take a digit at the end of the sentence in stockfish code style), and just copy-pasting your whole pull request message as the commit message.

Nice pull request message, by the way, thanks :-)

And after that,if want to test something of top of your change, please duplicate the branch: the branch must not change anymore if you are happy with the pull request state.

[31m059]

@snicolet Thanks!

[Rocky640]

You can replace
(pos.pieces(Them) ^ pos.pieces(Them, PAWN))
with
nonPawnEnemies

[snicolet]

Merged via 759d129, congrats :-)