Remove/improve backwards compatibility aliases

[shumpohl]

• qupulse.pulses.plotting: Replace pulses/plotting.py with lazy import in pulses/__init__,py
• AtomicPulseTemplate.atomicity: Remove
• qupulse/pulses/pulse_template_parameter_mapping.py: Remove pulse_template_parameter_mapping.py #913

I realized that there is "we will keep it around forever." in qupulse/pulses/plotting.py

[shumpohl]

I realized that there is "we will keep it around forever." in qupulse/pulses/plotting.py