Fix per60 features when columns are null

[michalekb11]

Options are to:

• Delete all affected rows from training set (next 10 games... not great when window is large)
• Remove the row where there is any null values, and backtrack to previous row where not null (in this case the "last 3 games" may not actually be last 3, but rather last 3 with a full complete row of data)