Pfhedge

* MAINT: Fix default params in CIR and Heston stochastic process (401) (Thank you, masanorihirano !)

* TEST: Add and refactor tests (400) (394)

* DOC: Clean documentation (396)

* DOC: Add `example_heston_iv.py` (403)

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* BUG: Fix BSAmericanBinaryOption (366) (391)

* TEST: Add more tests to BS modules (366) (391)

* MAINT: Rename base classes to "Base*" (close 384) (386)

* `Instrument`, `Primary`, `Derivative` are deprecated. Use `Base*` instead.

* MAINT: Miscellaneous maintainances (385) (387) (388) (389)

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* ENH: Add `entropic_risk_measure` to `nn.functional` (close 352) (372)

* ENH: Add `value_at_risk` to `nn.functional` (371)

* MAINT: Add typing (378)

* MAINT: Drop Python 3.6 (close 356) (357)

* Python 3.6 is no longer supported. Please update to >=3.7.

* CHORE: Support PyTorch 1.10 (377)

* CHORE: Update README.md (375)

* CHORE: Update pytest-cov requirement from ^2.8.1 to ^3.0.0 (367)

* CHORE: Update Makefile (381)

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* MAINT: Fix buffer key (353)

* MAINT: Fix prevention of zero division (359)

* DOC: Miscellaneous updates (358) (360) (361) (362)

* CHORE: Run CI and building documentation in Poetry environment (363)

* TEST: Fix typo in test of spot (354)

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* ENH: Add features `spot` and `underlier_spot` (345)

* ENH: Additional Features feature (343)

* MAINT: Update repr of clauses (342)

* DOC: Minor updates (339) (340) (344)

* DOC: Fix no-transaction band class for new implementation (346)

* CHORE: Measure test coverage for Python 3.9 (347)

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* ENH: Add `Derivative.add_clause()` (close 328) (330)

* See [`examples/example_knockout.py`](https://github.com/pfnet-research/pfhedge/blob/0.13.0/examples/example_knockout.py) for an example to add a knock-out clause.

* MAINT: Prefer `feature.get` over `feature.__getitem__` (324)

* MAINT: Deprecate specifying derivative dinfo (331)

* MAINT: Minor refactoring (329)

* DOC: Minor updates (332) (325)

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