replace DataFrame.append for pandas.concat for pandas 2 compatibility

[fwaibl]

Description

I noticed that the tests are currently failing due to an incompatibility with pandas version 2. The solution is to use pd.concat instead of DataFrame.append.

Todos

Similar problems might arise in other files, but it is hard to find them reliably. I found the object NOE_violations in files/otherfiles/noe_output.py

[codecov[bot]]

Codecov Report

Merging #309 (4bdb3c0) into main (f0f8c14) will increase coverage by 0.54%. The diff coverage is 90.90%.

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
+ Coverage   54.40%   54.94%   +0.54%     
==========================================
  Files          93       93              
  Lines       14349    14339      -10     
==========================================
+ Hits         7806     7879      +73     
+ Misses       6543     6460      -83     

Flag	Coverage Δ
unittests	54.94% <90.90%> (+0.54%)	:arrow_up:

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

Impacted Files	Coverage Δ
pygromos/files/trajectory/trc.py	88.25% <75.00%> (ø)
pygromos/files/otherfiles/noe_output.py	61.86% <93.33%> (+61.86%)	:arrow_up:
pygromos/files/coord/cnf.py	45.36% <100.00%> (ø)
pygromos/files/trajectory/_general_trajectory.py	65.76% <100.00%> (ø)