Add copyright headers

Dear @karamolegkos ,

We analysed the current code and found that copyright headers were missing from the majority of text files. We have produced a Merge Request to show mainly the type of headers we expect when IP Reviewing open source code.

We invite you to check each change because there are some cases in which the addition of the header can create issues, like in html files containing cut code for other files, or like txt files we are not sure if comments or a header are expected. Please have a look at them.

You'll notice we've assigned ownership to "University of Piraeus Research Centre". If this is not the correct name or owner, please let me know and I will correct it. Use this Merge Request issue for this.

You will notice as well that in each file there is a placeholder for the Contributors (last section in the header). Feel free to add your name and contribution, or other's names there. I have noticed your name in some of the files, this Contributors section is the correct location to put your name 😄

Feel free to be inspired and/or accept the whole PR.

Thanks a lot

/cc @pkriefrvi @asgomes