Replace hardcoded variables

[bjrne]

Find a solution for currently hardcoded variables. This includes variables like _C2_B5, genome_length etc. Hardcoding them in the blocks' JS strings is ugly and could lead to errors:

• If these are not already in a preloaded example workspace they might not exist.
• If a user declares a function or something else with the same name, the variable might be renamed by blockly to avoid a collision