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.