Add `OVER_50` to the AGB age enum

[jatkinson1000]

As pointed out in #103 whilst 50+ is the WA and AGB syntax, Over 50 would be more compatible with junior ages (Under N).

Add this as an enum mapping to 50+, similar to how the open category is proposed to be handled in #152

[TomHall2020]

Big fan - if that let us add aliases to bypass the redundant AGE_ prefix from the age enum (so AGB_ages.ADULT instead of AGB_ages.AGE_ADULT at the same time I'd love it even more so - always bugs me that that has to be there. If I remember right and it really was just because AGB_ages.50_Plus is syntactically invalid, the enum member couldn't start with a number.

Figure as the enum is a developer utility most people would prefer the shorter keystrokes and can mange mentally mapping to the official terminology.

[jatkinson1000]

Yes, I also always disliked this.

My issue, and I've just carefully read the WA rules (§4.2) and drawn it out, is that "Over 50" for the 50+ category is semantically incorrect when compared to the junior categories.

E.g. for juniors:

"An athlete may participate in a Under 18 Class in tournaments when the competition takes place up to and in the year of the athlete's 17th birthday."

For consistency an Over 50 would read:

"An athlete may participate in a Over 50 Class in tournaments when the competition takes place in the year after that of the athlete's 50th birthday."

However, it does not, instead reading:

"An athlete may participate in a 50+ Class in tournaments when the competition takes place in the year of the athlete's 50th birthday and thereafter."

So 50+ and Over 50 are semantically different.

However, I also feel that most people wouldn't think about it this hard and "know what it means" and it would make the code much nicer...

[jatkinson1000]

@TomHall2020 I opened #159 to implement this.

Any thoughts on that and the above appreciated.