Implementation of lowering and some derivatives for TwoBand

[codecov[bot]]

Codecov Report

Base: 92.83% // Head: 93.02% // Increases project coverage by +0.19% :tada:

Coverage data is based on head (771da2f) compared to base (fd8ee8e). Patch coverage: 94.11% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #71 +/- ## ========================================== + Coverage 92.83% 93.02% +0.19% ========================================== Files 4 4 Lines 670 703 +33 ========================================== + Hits 622 654 +32 - Misses 48 49 +1 ``` | [Impacted Files](https://codecov.io/gh/JuliaApproximation/SemiclassicalOrthogonalPolynomials.jl/pull/71?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) | Coverage Δ | | |---|---|---| | [src/twoband.jl](https://codecov.io/gh/JuliaApproximation/SemiclassicalOrthogonalPolynomials.jl/pull/71?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL3R3b2JhbmQuamw=) | `95.34% <94.11%> (-0.49%)` | :arrow_down: | | [src/SemiclassicalOrthogonalPolynomials.jl](https://codecov.io/gh/JuliaApproximation/SemiclassicalOrthogonalPolynomials.jl/pull/71?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation#diff-c3JjL1NlbWljbGFzc2ljYWxPcnRob2dvbmFsUG9seW5vbWlhbHMuamw=) | `88.23% <0.00%> (+0.39%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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[ioannisPApapadopoulos]

@dlfivefifty happy to merge this?

[dlfivefifty]

I suspect its a very inefficient way of doing this (specifying the data matrix directly via _BandedMatrix is generally preferred) but it can be merged for now