feature: add layer concept

[codecov[bot]]

Codecov Report

Patch coverage: 94.84% and project coverage change: -0.20% :warning:

Comparison is base (c43a359) 95.80% compared to head (978bd63) 95.61%. Report is 25 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #45 +/- ## ========================================== - Coverage 95.80% 95.61% -0.20% ========================================== Files 17 18 +1 Lines 1861 2031 +170 Branches 653 653 ========================================== + Hits 1783 1942 +159 - Misses 48 59 +11 Partials 30 30 ``` | [Files Changed](https://app.codecov.io/gh/abess-team/skscope/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team) | Coverage Δ | | |---|---|---| | [skscope/base\_solver.py](https://app.codecov.io/gh/abess-team/skscope/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team#diff-c2tzY29wZS9iYXNlX3NvbHZlci5weQ==) | `94.25% <91.93%> (-0.88%)` | :arrow_down: | | [skscope/solver.py](https://app.codecov.io/gh/abess-team/skscope/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team#diff-c2tzY29wZS9zb2x2ZXIucHk=) | `96.87% <95.00%> (-0.54%)` | :arrow_down: | | [skscope/layer.py](https://app.codecov.io/gh/abess-team/skscope/pull/45?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=abess-team#diff-c2tzY29wZS9sYXllci5weQ==) | `96.39% <96.39%> (ø)` | |

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