Create a vignette dedicated to importing data to scp

[cvanderaa]

• [x] Show required tables and how to generate them
• [x] Use standard formats. For instance, use an untouched MaxQuant evidence table (see issue #21). Other examples could show how to import PD files, metamorpheus files, (other?)...
• [x] Describe the required fields in the table to properly run a workflow with scp
• [x] Show different use cases, eg creating a QFeatures object from a SingleCellExperiment object

(feel free to add/suggest more items)

[cvanderaa]

Done since https://github.com/UCLouvain-CBIO/scp/commit/e56e63d1bf1c473a2b24ea1b2de4c37f8443ed1b