ISIS-621: further improvements to BigDecimal parsing

* prohibit grouping (thousands) separator when enter value
* render thousands separator in view mode
* show decimal places up to scale (eg 123.40 for scale=2)

Implementation notes:
* JavaMathBigDecimalPanelFactory now caches BigDecimalConverterWithScale per scale,
  since each will have different behaviour but is still safe to
  be shared across threads.