detect large number of significant digits

[ThierryO]

Storing floating point numbers with a large number of significant digits is often not relevant. The large number of significant digits might lead to irrelevant diffs. E.g. when the floating point is the result of an analysis and the value changes only a tiny bit to due randomness in the analysis.

• allow to specify the number of significant digits. E.g. via write_vc() or update_metadata()
• issue a warning when using write_vc() without specified number of significant digits.
• automatically round the floating points when the number of significant digits is specified.