Fix #92priv: [Bug] Volume DF DCV bounds (#809)

Fix df-private#92: [Bug] Volume DF's DCV bounds are per user & per-asset, should only be per-asset

- wrote a special test to catch the issue: `test_bound_by_DCV_1nft_2accounts()`. The test was failing (as it should have been)
- then fixed the issue: just one line in df_py/volume/reward_calculator.py. This made the failing test pass (good)
- also, a couple other tests in df_py/volume/test/test_calcrewards.py were failing: test_volume_reward_calculator() and test_volume_reward_calculator_predictoor_mul(). I refactored them to be more compact, then fixed the failures

Author: trentmc

df_py/volume/reward_calculator.py

@@ -201,7 +201,7 @@ def _calc_rewards_usd(self) -> np.ndarray:
                 R[i, j] = min(
                     perc_at_j * perc_at_ij * self.OCEAN_avail,
                     ocean_locked_ij * TARGET_WPY,  # bound rewards by max APY
-                    DCV_OCEAN_j * multiplier,  # bound rewards by DCV
+                    DCV_OCEAN_j * perc_at_ij * multiplier,  # bound rewards by DCV
                 )
 
         # filter negligible values