Add new isatab template 'cancer'

[Thomas]

Discussed this with @mbenary. Current templates tumor_normal_dna & *_triplets are unsatisfactory in multiple ways, not the least by relying on external config injection for "triplets":

cubi-isa-templates/cubi_isa_templates/__init__.py

Line 108 in 89d1a02

 configuration=load_variables("isatab-tumor_normal_triplets", {"is_triplet": True}), 

We therefore decided to start a better named and hopefully more useful singular cancer template.

To do

• copy tumor_normal_triplets files as a basis
• adapt is_triplet logic to selectively add transcriptomics assay within the template
• add human readable prompts
• get feedback from @mbenary & @ericblanc20,
• discuss addtional changes (i. e. removing not used protocol parameters)
• remove tumor_normal_* templates?