Fix ATR calculation for inputs shorter than the window

[aerraj]

Summary

Fixes #236.

Only initialize the ATR seed when the input contains a complete window. Previously, an empty or short input indexed past the end of the output array and raised IndexError.

This retains the existing zero-valued warm-up convention: a series shorter than the window returns zeros on its original index. It does not change the warm-up policy to NaNs, alter the ATR recurrence, or change results once the window is complete.

Tests

• Added checks for empty, one-row, and five-row inputs with a six-row window, both fillna settings, and both class/function APIs.
• Added exact-window and one-extra-row checks to verify seeding and recurrence remain unchanged.
• Before the fix, all six short-input subcases raised IndexError.
• python -m unittest test.test_atr_short_input test.unit.volatility: 56 tests passed.
• Black 23.10.1 (the version in requirements-test.txt) on both changed files and git diff --check: passed.

The full python -m unittest discover run completed 136 tests with two unrelated errors in TestTSIIndicator.test_tsi and test_tsi2: pandas removed the check_less_precise assertion argument. A pristine upstream worktree reproduces the same two errors among its 134 tests. No TSI code or tests were changed.

Environment: Python 3.12, pandas 2.3.3, NumPy 1.26.4. All tests use local data.

Upstream CI status

The CircleCI Python 3.6 job fails during Install dependencies, before lint or tests run: No matching distribution found for numpy==1.21.5. The existing configuration uses Python 3.6 with that NumPy pin. This PR does not modify the CI image or dependency requirements.