Add copyright headers

Dear @imurua ,

We analysed the current code and found that copyright headers were added to many source code files, but they were missing in some of the 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 files which are input for another process and we are not sure if comments or a header are expected. Please have a look at them.

We have followed the same type of headers you have already added yourself. You will notice that we have not added the placeholder for the Contributors in the last part of the copyright header. Normally it's a good practice to add that section so that the contributors can add their name and contribution (Idoia Murua - Author, for instance).

Last but not least, I would like to mention that it's important to add the SPDX identifier in the header, as the automatic tools will read that. In the headers you have added the SPDX identifier is not present, I recommend you to add a line at the end of each header with this information. In the case of MIT license, the line would be:

SPDX-License-Identifier: MIT

I hope it helps. Feel free to be inspired and/or accept the whole Merge Request.

Thanks a lot

/cc @asgomes