fix bug in laplace labels

[GiovanniCanali]

Fixes #523

Checks:

• Does the same problem affect other differential operators? Edit:: Yes.

To do:

• Cast single string, passed to either components or d attributes, to a list of a strings (length 1) to avoid issues when the string length is greater than 1.
• Update the documentation: d and components accept as input type both str and list[str].
• Improve tests to manage labels as strings of different lengths.
• Remove useless if-else structure in laplacian: the "if" case is handled also in the "else" case.

[dario-coscia]

Great for me!