allow shorting stocks in US stock market

[2young-2simple-sometimes-naive]

update position.py to allow short orders (implemented as negative amount in position).

Description

Allow negative amount in position to represent short orders in US stock.

Motivation and Context

Allow short orders will allow more flexible strategies.

How Has This Been Tested?

• Pass the test by running: pytest qlib/tests/test_all_pipeline.py under upper directory of qlib.
• If you are adding a new feature, test on your own test scripts.
  tested with xgboost script.

Screenshots of Test Results (if appropriate):

1. Pipeline test:
   pytest.out.txt

2. Your own tests:
   workflow-xgboost.log.txt

Types of changes

• Fix bugs
• Add new feature
• Update documentation

[you-n-g]

@2young-2simple-sometimes-naive
Thanks for your PR.
But shorting is not that simple.
More factors should be considdered (e.g. margin trading)

In this PR, the cache will increase when we short stock.

Thanks