Add support for parametrized pulses

[goerz]

This adds support for parametrized pulses by attaching an optional 'parametrization' attribute to control-functions and pulse arrays. The parametrization object stored in that attribute contains information for converting ϵ⟷u and calculating the derivative ∂ϵ/∂u. These are automatically taking into account when calculating the pulse updates (μ = (∂Ĥ/∂ϵ)⋅(∂ϵ/∂u) for an Ĥ that is linear in ϵ).

Thus, all internal storage is for the physical pulse values ϵ(t), and any conversion to u(t) is done on the fly as necessary. This ensures that everything stays 100% compatible with QuTiP, but adds a small overhead from the ϵ⟷u conversions compared to the "hacky" solution discussed in https://github.com/qucontrol/krotov/issues/23#issuecomment-850887732. The latter could still be used by an advanced user when performance is more critical than clarity.

Closes #23

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Codecov Report

Merging #92 (809d94e) into master (cfe6b10) will decrease coverage by 2.8%. The diff coverage is 45.4%.

@@           Coverage Diff            @@
##           master     #92     +/-   ##
========================================
- Coverage    96.1%   93.2%   -2.9%     
========================================
  Files          13      14      +1     
  Lines        1676    1765     +89     
========================================
+ Hits         1611    1646     +35     
- Misses         65     119     +54     

Impacted Files	Coverage Δ
src/krotov/__init__.py	100.0% <ø> (ø)
src/krotov/mu.py	90.0% <33.3%> (-10.0%)	:arrow_down:
src/krotov/parametrization.py	36.1% <36.1%> (ø)
src/krotov/conversions.py	95.6% <75.0%> (-2.5%)	:arrow_down:
src/krotov/optimize.py	96.7% <75.0%> (-1.0%)	:arrow_down:

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