Adds the amount to the units available skill points. Calling with a negative
number reduces the skill points by that amount.
Returns false if the amount of available skill points is already zero and
if it's called with any non-positive number.
Returns true in any other case.
If skillPointDelta is greater than the amount of skillpoints the hero
actually can spend (like 9 for three 3-level abilities) only that amount will
be added. Negative skillPointDelta works as expected.
Adds the amount to the units available skill points. Calling with a negative number reduces the skill points by that amount. Returns false if the amount of available skill points is already zero and if it's called with any non-positive number. Returns true in any other case.