With --json, --grad should not accept extra argument

[foxtran]

Fixes #97

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 50.22%. Comparing base (3a50cbd) to head (e68f93f).
Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
app/cli.f90	0.00%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
+ Coverage   48.63%   50.22%   +1.58%     
==========================================
  Files          34       24      -10     
  Lines        3707     2449    -1258     
  Branches     1518      988     -530     
==========================================
- Hits         1803     1230     -573     
+ Misses       1207      878     -329     
+ Partials      697      341     -356     

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[awvwgk]

I don't think this is an intuitive feature to implement as the behavior of --grad would depend on whether it comes before or after the --json argument.

With this PR

s-dftd3 test.xyz --json --grad test.txt

would fail because of the extra argument but

s-dftd3 test.xyz --grad test.txt --json

would be fine.