Class level tag CAST and output token SPELLLISTCAST need new way to indicate at will spellcasting

Some classes have at will spellcasting for certain levels, for example, spontaneous casters with cantrips in Pathfinder. PCGen uses "0 spells per day of that level" as an indicator for "at will", and the output sheet tests for spell levels that have spells known not equal to 0, but cast per day equal to 0 and recognizes that as at will spell casting.

However there are cases where classes can actually have "0 spells per day" of a certain level, for example, at the level when that spell level is first gained. In most cases, the character will have a high enough casting attribute, so that he gains bonus spells, and thus has more than 0 spells per day at the level. However, it is possible that the character does not have a high enough casting attribute, and this has 0 spells per day at the newly gained spell level, or the game mode might not even allow bonus spells.

This was the topic of issue OS-307, which I fixed with the workaround to use "at will" only for 0 level spells, but that my not always be right.

Thus we need a new way to indicate at will spellcasting in the CAST tag. One possibility would be to use "-1" instead of "0", but that would still need code work as the CAST tag allows only non-negative integers.

I would rather propose to change the CAST tag to allow text as well, with ATWILL being the only allowed value for now.

The SPELLLISTCAST output token should then also be able to return an appropriate new value, such as ATWILL that can be tested for in the output sheet.