Reduce capacity of polygon_{;45_;90_}set with shrink_to_fit functionality

[sbigalke]

First, I'd like to thank you for providing such an awesome library.

But I think the method shrink_to_fit() is missing in order to reduce the capacity of any set.

In your documentation, it is mentioned under the clear() command that one should

Use shrink to fit idiom and assign default constructed polygon set to deallocate.

but there is no way of getting the vector to "shrink_to_fit()" because the value() function is declared as const.
So basically, one has no option to deallocate the memory of a huge polygon_set .

An easy fix would be to add a method as follows:

void shrink_to_fit(){
    data_.shrink_to_fit();
}