text: Title: Possible Bug in the Center of Gravity Indicator
  Body: When I ran `Center of Gravity: cg` over 3 months of Bitcoin prices ("20200801" to "20201101"), I got
  
  | | Close | cg | 
  | -- | -- | -- |
  | count | 132481.000000 | 132472.000000 |
  | mean | 11378.306788 | -5.499988 |
  | std | 844.355621 | 0.001991 |
  | min | 9881.820000 | -5.616297 |
  | 25% | 10710.500000 | -5.500833 |
  | 50% | 11368.680000 | -5.499987 |
  | 75% | 11742.540000 | -5.499146 |
  | 100% | 14068.490000 | -5.458667 |
  
  After reading http://www.mesasoftware.com/papers/TheCGOscillator.pdf, I believe there should be both positive and negative values, as well as a fair bit of variance. Based on the equations and pseudo code given in the pdf, I think a `.sum()` at the end of the following line, https://github.com/twopirllc/pandas-ta/blob/development/pandas_ta/momentum/cg.py#L42, is needed.
  
  Thanks again for the amazing library!