Introduce constants for context params

[stokito]

Across the all code it's a common practice to use Map-like contexts. Usually the contexts are used to store some param via .set(paramName, paramValue) method. For example:

  AmountFormatQueryBuilder.of(Locale.GERMANY)
    .set(AmountFormatParams.PATTERN, "####,####")
    .build()

Here we called .set(AmountFormatParams.PATTERN, "####,####") and the PATTERN constant is just a string "pattern". But sometimes instead of constant it's used a raw string i.e. .set("pattern", "####,####"). It will be little bit better to use constants: we can document them and easily find all usages and avoid typos. You try to search all callings of .set(" in code base and here is short list of my findings:

• AmountFormatQueryBuilder
  • strict, omitNegative, omitNegativeSign: they are mentioned in userguide.adoc but not used anywhere in sources.
• RoundingQueryBuilder
  • cashRounding, scale, minimalMinors: are used in code as a raw string but for all of them there is a constants inside of org.javamoney.moneta.internal.DefaultCashRounding but the constants are private. Maybe we can make them public?
• MonetaryContextBuilder
  • MonetaryRounding
• ProviderContextBuilder
  • providerDescription, days

[acelopezco]

I suppose this should be moved to the issues page of jsr354-api project ?