Remove drum modifiers

The modifiers

• Drum::set_coupling_0,
• Drum::set_coupling_1,
• Drum::set_coupling_2,
• Drum::set_drive

are redundant. They should not be used anymore and code should be migrated towards utilizing the reference-access members instead. I believe this carries no performance impact, but this should be tested first.

For now, any DrumVariables-type class should implement the four public data members

• t0,
• t1,
• t2,
• V,

even if they are not utilized.

Note that System has similar requirements in its private implementation.

Given that this structure resembles actual experiments, at least the member enforcement should probably stay.