+ * Formats a given value with the specified SI prefix.
+ * @param v The value to format.
+ * @param unit The unit of quantity.
+ * @param prefix The number of the prefix, from 0 for 'femto' up to
+ * 8 for 'giga'. If prefix is set to -1, the prefix will be calculated.
+ * @param precision The number of digits after the decimal separator.
+ * @param sign Whether or not to add a sign also for positive numbers.
+ *
+ * @return The formated value.
+ */
+QString format_si_value(
+ double v, QString unit, int prefix = -1,
+ unsigned precision = 0, bool sign = true);
+
+/**
+ * Formats a given time with the specified SI prefix.