====== ChangeHeroStat() ======

===== Syntax =====
''ChangeHeroStat(heroName, statID, delta);''

''heroName'' – the hero's name
''statID'' – the stat's ID
''delta'' – stat modification value

===== Description =====
This function modifies the hero's specified stat by ''delta''. The following stats can be modified: experience, attack, defense, spell power, knowledge, luck, morale, move points, mana points.
''delta'' can take on either positive (the stat is increased) or negative (the stat is decreased) values for all stats except experience, since experience can only grow.

The stats' values themselves can not be negative. The top values of move points and mana points stats are additionally limited by the move points max and mana points max values. If the modification results in the stat exceeding the permissible limitations, it is truncated.

''statID'' can take one of the following values:
  * ''STAT_EXPERIENCE'' – the hero’s experience
  * ''STAT_ATTACK'' – attack level
  * ''STAT_DEFENCE'' – defense level
  * ''STAT_SPELL_POWER'' – spell power
  * ''STAT_KNOWLEDGE'' – knowledge
  * ''STAT_LUCK'' – luck
  * ''STAT_MORALE'' – morale
  * ''STAT_MOVE_POINTS'' – remaining movement points number
  * ''STAT_MANA_POINTS'' – current mana points number