Add `version.m` file to the library

[dcajal]

Description:

We want to include a simple, programmatic way for users (and developers) to check which version of biosigmat is installed.

This is useful for:

• Debugging and support.
• Ensuring compatibility between components.
• Tracking installations across different systems.
• Matching the version with GitHub releases.

Implementation:
We’ll add a file at src/version.m with the following content:

function v = version()
    v = '0.1.0';  % Must match the corresponding GitHub release tag
end

Tasks:

• Create the version.m file inside src/.
• Add usage instructions to the README:

biosigmat.version()

Note:
This version string must be updated each time a new GitHub release is published.