Pfhedge

* API: Unify to `init_state` (close 189) (193)

* API: Add `Derivative.ul()` as an alias to underlier (close 183) (195)

* MAINT: Introduce and tame mypy (close 190) (191)

* MAINT: Add type check to CI (close 190) (192)

* MAINT: Refactor generate_cir (194)

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* BUG: Fix invalid time series of CIR process (close 182) (186) (Thank you, masanorihirano !)

* DOC: Add missing `dt` to `generate_cir` (186)

* DOC: Update an example in README.md (181)

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* DOC: Mention simulation method of CIR and Heston processes 173 (174)

* DOC: Add reference to BS price formula 129 (175)

* DOC: Fix typo in BSEuropeanOption docstring (close 164) (177)

* DOC: Add equations to docstrings of brownian motion (close 138) (178)

* ENH: Add `moneyness()`, `log_moneyness()`, `time_to_maturity()` to Options (close 165) (167)

* ENH: Fix default init state of `HestonStock` (161)

* GITHUB: Fix GitHub Actions (162)

* REF: Add typing and refactor (163) (160)

* ENH: Add `Instrument.cpu()` (close 144) (150)

* ENH: Add `Instrument.cuda()` (close 145) (151)

* ENH: Add `Instrument.float()` (close 146) (153)

* ENH: Add `Instrument.double()` (close 147) (154)

* ENH: Add `Instrument.half()` (close 148) (155)

* ENH: Add `Instrument.bfloat16()` (close 149) (156)

* BUG: Fix 140 (141)