change strings to char array for old matlab compatibility

[aleruggeri87]

My MATLAB R2017b supports strings, but it seems the set function doesn't handle them well.
To work around this, I used char arrays for parameters.
I assume this workaround is compatible with newer versions as well, considering that char arrays are used elsewhere in the code.

Best,
Alessandro.

[bastibe]

Thank you for the pull request.

This seems innocuous enough, but we should verify that this doesn't break anything on newer versions of Matlab. I don't have access to Matlab any longer. Could you, or somebody else, cross-check that this works on a recent Matlab?

[aleruggeri87]

The current set documentation page is somewhat vague regarding the types of parameters it accepts. While the examples primarily use strings, there are instances where character arrays are also used.
Since I do not have access to the latest version of MATLAB, I asked around and found a colleague who has R2021a. He tested the set function with character arrays, and it is still working in that version.
Maybe some other user can help by trying with newer versions.

Cheers,
Alessandro.

[bastibe]

Thank you very much, then!