Find a way to format percentage intervals

[philsf]

The current state of affairs prevents the use of format.pct in format.interval, because format.float returns "NA" for both ends of the interval.

Possible solutions

• include an argument in format.interval to consider the interval as proportions
• include a check in format.float to see if the last char is "%"

[philsf]

With the solution to #40 , now format.interval should work by default on percentages, unless the pct.symbol is manually defined in the argument setting.

Consider if this is sill a valid issue.

[philsf]

#49 means that it is more important to solve this issue here, than to change every point where the function has been used before.