Replace headers with MIT license headers

Dear @jhimanen ,

After the license change, I create this Merge Request to replace the copyright headers and include the MIT ones.

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.

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

Thanks.