Add the possibility to use a different file indexing for the mat files

[GiulioRomualdi]

With this PR it will be possible to support different methods for file indexing.

For instance setting

config.file_indexing = "%Y_%m_%d_%H_%M_%S";

you will get a file named <file_name>_2021_12_13_19_42_08.mat

cc @Nicogene @AlexAntn @traversaro @S-Dafarra

[GiulioRomualdi]

Hi @Nicogene, I updated the example and the README.

[Nicogene]

Merged, thanks!