Fix sign conversion issues

[define-private-public]

I was including this on a project of mine where I had -Wall -Wextra -pedantic turned on to clean up any extra warnings. I caught about 4 of them in relation to implicit sign and type conversions. Adding some static_cast<> calls and changing a data type resolved all of the warnings.

For reference, here are the warnings:

ProgressBar.hpp: In member function ‘void progresscpp::ProgressBar::display() const’:
ProgressBar.hpp:29:42: warning: conversion to ‘float’ from ‘unsigned int’ may alter its value [-Wconversion]
         float progress = (float) ticks / total_ticks;
                                          ^~~~~~~~~~~
ProgressBar.hpp:30:26: warning: conversion to ‘float’ from ‘unsigned int’ may alter its value [-Wconversion]
         int pos = (int) (bar_width * progress);
                          ^~~~~~~~~
ProgressBar.hpp:37:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
         for (int i = 0; i < bar_width; ++i) {
                         ~~^~~~~~~~~~~
ProgressBar.hpp:37:29: warning: conversion to ‘unsigned int’ from ‘int’ may change the sign of the result [-Wsign-conversion]
         for (int i = 0; i < bar_width; ++i) {