[point] Implement a Point system

[joelalejandro]

Points are cumulative units that can reflect different features of a creature.

Some examples would be hit points (how many hits a creature can take before dying or being destroyed), magic points (how much magical energy a creature is holding) or experience points (how much has a creature progressed in its life, raising its "level" and therefore other points).

• Points have a denomination.
• Points can be increased or decreased arbitrarily ([point] Implement a way for arbitrarily increasing/decreasing the score counter of a Point #37)
• Points can be the result of a calculation of several depending variables, such as ability score modifiers ([point] Allow calculation of the score counter to be computed from depending variables #38).

[joelalejandro]

Necessary for #28.