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Load .xdf files with gaps correctly

4

Fix [#385](https://github.com/cbrnr/mnelab/issues/385). Correctly load xdf files with incomplete data/gaps by inserting nan for missing data instead of interpolation.

Correctly load marker streams from .xdf files

2

Fixes [#451](https://github.com/cbrnr/mnelab/issues/451) in order to load XDF containing multi-channel marker stream. Also addresses some minor issues to recognize different names of marker types and auto-convert stream dtype to supported types...