Add Multiset

[sak-codes]

Fixes #390

[codecov[bot]]

Codecov Report

Merging #538 (1205740) into main (c7b2af7) will increase coverage by 0.016%. The diff coverage is 100.000%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #538 +/- ## ============================================= + Coverage 98.586% 98.602% +0.016% ============================================= Files 32 33 +1 Lines 4173 4222 +49 ============================================= + Hits 4114 4163 +49 Misses 59 59 ``` | [Impacted Files](https://app.codecov.io/gh/codezonediitj/pydatastructs/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codezonediitj) | Coverage Δ | | |---|---|---| | [...astructs/miscellaneous\_data\_structures/\_\_init\_\_.py](https://app.codecov.io/gh/codezonediitj/pydatastructs/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codezonediitj#diff-cHlkYXRhc3RydWN0cy9taXNjZWxsYW5lb3VzX2RhdGFfc3RydWN0dXJlcy9fX2luaXRfXy5weQ==) | `100.000% <100.000%> (ø)` | | | [...astructs/miscellaneous\_data\_structures/multiset.py](https://app.codecov.io/gh/codezonediitj/pydatastructs/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codezonediitj#diff-cHlkYXRhc3RydWN0cy9taXNjZWxsYW5lb3VzX2RhdGFfc3RydWN0dXJlcy9tdWx0aXNldC5weQ==) | `100.000% <100.000%> (ø)` | | | [pydatastructs/trees/binary\_trees.py](https://app.codecov.io/gh/codezonediitj/pydatastructs/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codezonediitj#diff-cHlkYXRhc3RydWN0cy90cmVlcy9iaW5hcnlfdHJlZXMucHk=) | `97.865% <100.000%> (+0.039%)` | :arrow_up: | [](https://app.codecov.io/gh/codezonediitj/pydatastructs/pull/538?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codezonediitj)