Newer Symbols have NaN values before Listing date

[miron]

Check all formulas and implement how to calculate with those DataFrames.
For now I limited calculations with [:3] to the first 3 symbols as those have no NaN values.
Possible solutions:

• limit calculations to 1 year into the past
• calculate individualy from first price, i.e. dropna()
• don't concatenate DataFrames and find another solution