WIP: Parametric types

[david-pl]

I have started addressing #189 here. There are still a lot of things to do in order to get to a fully type-stable point. The following points need to be done in order for this PR to be complete:

• Parametrically type Bra/Ket
• Parametrically type CompositeBasis
• Rename the abstract Operator type to AbstractOperator
• Replace SparseOperator and DenseOperator by a single Operator type with parametric fields
• Replace the gemv! and gemm! definitions by mul!
• Define a specialized type for density operators (?)
• Parametric typing for other operator types (Lazy, FFT, ...)
• Remove unnecessary checks (e.g. most IncompatibleBases errors will be covered by a MethodError
• Proper profiling and benchmarking

Going type stable will boost overall performance (though probably not by much). Also, this will add support for sparse density matrices, which can be very useful.

[coveralls]

Coverage decreased (-0.09%) to 99.477% when pulling 6aef249 on parametric-types into 4f7e1a8 on master.

[codecov]

Codecov Report

Merging #234 into master will decrease coverage by 0.09%.
The diff coverage is 96.15%.

@@            Coverage Diff            @@
##           master     #234     +/-   ##
=========================================
- Coverage   99.56%   99.47%   -0.1%     
=========================================
  Files          35       35             
  Lines        2325     2297     -28     
=========================================
- Hits         2315     2285     -30     
- Misses         10       12      +2

Impacted Files	Coverage Δ
src/schroedinger.jl	100% <ø> (ø)	⬆️
src/state_definitions.jl	100% <ø> (ø)	⬆️
src/timecorrelations.jl	100% <ø> (ø)	⬆️
src/phasespace.jl	100% <ø> (ø)	⬆️
src/stochastic_master.jl	100% <ø> (ø)	⬆️
src/particle.jl	100% <ø> (ø)	⬆️
src/mcwf.jl	97.69% <0%> (-0.07%)	⬇️
src/printing.jl	98.87% <100%> (-0.02%)	⬇️
src/states.jl	95.55% <100%> (-4.45%)	⬇️
src/spectralanalysis.jl	100% <100%> (ø)	⬆️
... and 27 more

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