[ENH] adding back MrSEQL to sktime

[fkiraly]

Fixes #4296, adds back MrSEQL to sktime.

See discussion there regarding temporary removal and complications in soft dependency isolation which now should all be resolved.

[fkiraly]

Looks like it works out of the box - FYI @lnthach.

Noting, the mrseql package does not seem to have its dependencies set properly? It depends on numba, and will break without numba, but pip install mrseql does not install numba.

Not a problem, since sktime estimators manage their own dependencies and it's added there, but it would be an issue for direct users of the mrseql package.

[lnthach]

Thanks @fkiraly . I will look into the dependency issue. Though I didn't use numba when implementing MrSEQL. Probably because MrSEQL depends on sktime for its SFA implementation.

[fkiraly]

Probably because MrSEQL depends on sktime for its SFA implementation.

Yes, MrSEQL depends on sktime's SFA which in turn depends on numba (sktime itself does not depend on numba, it is a soft dependency of SFA)

[fkiraly]

hm - hold a moment, it does depend on numba only if symrep contains the "sfa" string!

This is an interesting case, as this makes the default configuration not numba or sktime dependent!

I think it is worth leaving the dependencies then, as thas allows users to use part of SFA without numba, which comes with a significant installation and maintenance overhead, and instead raising an error message if SFA is used (which already happens afaik).