Calculate maximum threat for hanging pieces

Use the max_threat() helper function to estimate more precisely the
best hanging piece threat.  Also retunes the Threat array using SPSA.

STC
LLR: 2.95 (-2.94,2.94) [-1.50,4.50]
Total: 7598 W: 1596 L: 1468 D: 4534

LTC
LLR: 2.97 (-2.94,2.94) [0.00,6.00]
Total: 7896 W: 1495 L: 1350 D: 5051

Bench: 6816504

Resolves #73
Reformat max_threat()

Helper function should just know how to find the
biggest piece type in a bitboard. All the threat
logic and data shoud be in evaluate_threats().

This nicely separates the scope of the two functions
in a more consistent way and simplifies the code.

No functional change.
Retire to_char() helpers

Remove some useless wrappers and make
the conversion explicit and starightforward.

No functional change.
Rename ucioption.h to uci.h

We are going to add all UCI related
functions here, so first rename it
to a more proper name.

No functional change.
Retire notation.h

And move the few remaining content
under UCI namespace where they belong.

No functional change.
Final UCI helpers renaming

To reflect new changes, specifically that
now are all under UCI namespace.

No functional change.
Retire notation.cpp

Now we can finally retire notation.cpp
and move UCI helpers under uci.cpp

No functional change.
Fix an obscure gcc warning

warning: narrowing conversion from ‘int’ to ‘char’ inside { }
is ill-formed in C++11 [-Wnarrowing]

When pedantic meets esoteric!

No functional change.
Improve compatibility with Shredder Classic GUI

This commit fixes two issues:

1) Don't print PVs after the search has been interrupted

    This solves the "mate 0 upperbound" scores that sometimes
    creep up when a multi-PV analysis gets interrupted with
    the `stop` command.

2) Print multipv before score

    Shredder Classic fails to identify the main PV
    (the one with multipv 1) if `score` comes first.
    This leads to an eval graph that doesn't reflect
    the scores actually reported by Stockfish when
    doing a multiPV analysis.

No functional change

Closes #76
Tune PawnsFileSpan

Passed the following SPRT tests:

STC:
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 24428 W: 5056 L: 4880 D: 14492

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,4.00]
Total: 26590 W: 4715 L: 4472 D: 17403

Bench: 6615949

Resolves #78