ENH: Warn on initial cash lower than Bitcoin price

Refs:
* kernc#134
* kernc#116
* kernc#151

Author: kernc

backtesting/backtesting.py

@@ -1057,6 +1057,11 @@ def __init__(self,
             raise ValueError('Some OHLC values are missing (NaN). '
                              'Please strip those lines with `df.dropna()` or '
                              'fill them in with `df.interpolate()` or whatever.')
+        if np.any(data['Close'] > cash):
+            warnings.warn('Some prices are larger than initial cash value. Note that fractional '
+                          'trading is not supported. If you want to trade Bitcoin, '
+                          'increase initial cash, or trade μBTC or satoshis instead (GH-134).',
+                          stacklevel=2)
         if not data.index.is_monotonic_increasing:
             warnings.warn('Data index is not sorted in ascending order. Sorting.',
                           stacklevel=2)