Add `ctx = SessionContext()` to __init__

[deanm0000]

That way there'd just be one line

from datafusion import ctx, col, lit, functions as F

[timsaucer]

Is this necessary? Now that we have the global context for reading files to create dataframes, this is potentially creating conflict of floating around two different default-ish contexts.

[timsaucer]

Closing since there is no additional feedback and we have the global_ctx already available.