ENH: Environment variable to set the Xoscar temp directory.

[MarcelHoh]

Following the discussion on my question on the slack channel, this PR adds the ability to set an environment variable for the xoscar temp directory. I've just called it XOSCAR_DIR for now. If the variable is not set the old behaviour of using the home directory is maintened.

[qinxuye]

Seems there are some format issue, we use black to format codes, you can run black xoscar.

[codecov]

Codecov Report

Merging #58 (c7754a4) into main (a868b1c) will increase coverage by 0.32%.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head c7754a4 differs from pull request most recent head 56a37df. Consider uploading reports for the commit 56a37df to get more accurate results

@@            Coverage Diff             @@
##             main      #58      +/-   ##
==========================================
+ Coverage   93.46%   93.78%   +0.32%     
==========================================
  Files          47       47              
  Lines        3671     3671              
  Branches      708      708              
==========================================
+ Hits         3431     3443      +12     
+ Misses        160      152       -8     
+ Partials       80       76       -4     

Flag	Coverage Δ
unittests	93.65% <100.00%> (+0.27%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
python/xoscar/constants.py	100.00% <100.00%> (ø)

... and 5 files with indirect coverage changes

[qinxuye]

LGTM