Implement Fields annotation

[Perwein]

Hey!

I implemented this because I am using fixedformat4j in a project. If you are interested, it would be nice to get it merged upstream. Looking forward for your comments and suggestions!

[maxsap]

+1

[shifuddin-masud-sixt]

+1

[jeyben]

I will proably implement it as a @RepeatedField annotation instead.
Otherwise the @Fields annotation is overloaded in functionality and can be misconfigured to have both multiple @Field annotations AND an count. Hard to make clear for a user what happens.

The @Fields annotation works well, when the same data is in several (not necessary repeated right after eachother) places in the same row - while a @RepeatedField will work for fields right after eachother

[jeyben]

Second thought.
I think it is more clean to add it to the @Field annotation and validate it is added to a Collection or Array

[jeyben]

Implemented as a count property on the @Field annotation from version 1.6.0