Add qexpand functionality and cleanup scalings

[apkille]

This PR addresses several discussions in #55:

• prefactorscalings evolved from its original purpose in a rather messy way. Now it straightforwardly pulls out scalar factors in multiplication and tensor products as well as commutators and anticommutators.
• Introduced flattenop function that parses repeated addition, multiplication, and tensor products as a single expression. This prevents any ugly nested expressions for these operations.
• Introduced qexpand function that manually expands expressions containing quantum objects. It is built on top of SymbolicUtils utilities for rule-rewriting in a way similar to qsimplify.
• Created files utils.jl and linalg.jl and did some rearranging of the source code. For instance, it didn't make sense for dagger to be in predefined.jl where we define things such as Pauli operators and harmonic oscillator operators, so dagger was moved to linalg.jl.
• Added express.md (which was merged in #57) to docs.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 80.30303% with 26 lines in your changes missing coverage. Please review.

Project coverage is 70.10%. Comparing base (079ea5e) to head (0d66026).

Files	Patch %	Lines
src/QSymbolicsBase/linalg.jl	77.77%	12 Missing :warning:
src/QSymbolicsBase/utils.jl	82.85%	6 Missing :warning:
src/QSymbolicsBase/basic_ops_homogeneous.jl	83.33%	5 Missing :warning:
src/QSymbolicsBase/rules.jl	81.81%	2 Missing :warning:
src/QSymbolicsBase/basic_ops_inhomogeneous.jl	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #59 +/- ## ========================================== + Coverage 66.25% 70.10% +3.85% ========================================== Files 15 17 +2 Lines 646 649 +3 ========================================== + Hits 428 455 +27 + Misses 218 194 -24 ```

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[Krastanov]

looks good! Feel free to update the changelog / merge / release (or to postpone some of these steps if it is more convenient for your near term plans)