Metadata import: Hidden requirements, user unfriendly

[henninggaertner]

Issue description

The metadata import has hidden requirements which require previous knowledge. For example: it is not clear that the first column has to be "Sample" in order for the metadata to function correctly, e.g. in the t-test or ANOVA.
A description of the required format of the metadata .csv is necessary.

Example error

Example misinterpretation of imported metadata

Acceptance Criteria

• The method description highlights the column naming convention
• The method description outlines the expected format of the metadata group columns
• A novice user is given all the information to format his metadata file correctly