Add IsaBundle as a container for investigation, study, assay

[holtgrewe]

Is your feature request related to a problem? Please describe.
It currently is cumbersome to read/write a full ISA-tab folder as investigation, study, and assay must be handled separately.

Describe the solution you'd like
Introduce a IsaBundle container class that contains all.

Describe alternatives you've considered
N/A

Additional context
N/A