Add time dependent expressions arithmetics for atomic pulse templates

[shumpohl]

It is very convenient to multiply pulses with a time dependent expression where possible. A typical use case are modulated pulses required for resonant driving.

There is a first draft in #704 which is problematic because it cannot handle composite pulse templates correctly since the atomic parts of it have no knowledge about the "global" time.

This issue tracks the first step which is to support aritmetics for atomic pulse templates.

[shumpohl]

Implemented for atomic pts in #704