Implemented HP and EP starting values

The absolute or percentage starting value can now be set for either HP or EP.

Setting the absolute value is done with `start-value`, while setting the percentage is done with `start-percentage`. Both of these are attributes of the `max-hp` and `max-ep` tags (different from Tiger's changes, which had the starting EP a tag of its own).

The starting percentages are always a percentage of the upgraded max value. Thus, if a unit normally has 1000 max HP and an upgrade is in effect that increases its HP by 500, then setting the max hp to have a starting percentage of 0.5 will result in the unit starting with 750 HP (the result of (1000 + 500) * 0.5).

This is particularly useful because the default behavior is to have units start with their max HP, even if they have upgrades that boost HP. Thus, if you want your units to be created with a starting HP equal to their true max HP (with upgrades), set the percentage starting value to 1.0.

Note that `start-value` and `start-percentage` are mutually exclusive. You cannot use both at the same time. Attempting to do so will cause a runtime error.

I have tested various combinations of starting values and percentages with and without upgrades.

Example:

```xml
<!-- ... -->
<max-hp value="500" start-percentage="1.0" />
<max-ep value="1000" regeneration="5" start-value="500" />
<!-- ... -->
```

## Known issues

* Setting the starting HP to 0 causes the unit to be immortal. This actually may be a feature.