Fix warnings

[christophe-david]

This PR fixes several warning messages from mission module:

• deprecation of compute_TOW option (even if not set by user)
• incorrect way of setting values in pandas DataFrames.

[codecov[bot]]

Codecov Report

Merging #492 (377e3e4) into master (b5498d4) will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #492      +/-   ##
==========================================
+ Coverage   84.34%   84.37%   +0.03%     
==========================================
  Files         130      130              
  Lines        6005     6005              
  Branches      885      884       -1     
==========================================
+ Hits         5065     5067       +2     
+ Misses        784      783       -1     
+ Partials      156      155       -1     

Impacted Files	Coverage Δ
...ad/models/performances/mission/openmdao/mission.py	96.66% <ø> (+2.10%)	:arrow_up:
src/fastoad/models/performances/mission/mission.py	94.87% <100.00%> (+0.13%)	:arrow_up: