Minor cleanup

Support params alter using alterPaymentProcessorParams hook

Merge Deepak's hook addition

Update extra-marks.md 

Changing the text of the tutorial to use GC's "paid out" simulation instead of "submitted", per conversation in Mattermost channel.

Replace Contribution.create call with SQL for fixing date; reduce code that duplicates core; add test to determine core behaviour

Allow to define a specific priceFieldID on import

Improvements to documentation

Co-authored-by: Rich Lott <artfulrobot@users.noreply.github.com>

Ah yes! Thanks for that.

Co-authored-by: Rich Lott <artfulrobot@users.noreply.github.com>

Fix link to documentation

Thanks @wmortada !