Update cell line meta data class

[wxicu]

PR Checklist

• Referenced issue is linked
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes

• Metadata classes should load data more lazily Metadata classes should load data more lazily #478: __init__ loads only the ones that are requested on annotate. The remainig files are downloaded upon request of annotating. However, all the files are forced to be downloaded when initializing the lookup object. Thus it takes much more time to initialize the look up object if all the files haven't downloaded before. Do you probably have any other suggestions?
• Improve annotate docstring in cell line Improve annotate docstring in cell line #479: Add Cancerxgene in the docstring. Also, rename all the downloaded files and their corresponding variables to be more readable.

Technical details

Additional context

[Zethson]

Guess it's okay if everything is downloaded when a lookup object is created. That's just the original behavior so whatever.