Decoupling of temp0 and temp keywords

[JanPolena]

Setting the temp0 keyword within the input.in file is now mandatory, unless the initial velocities are provided either from an external file or by restarting the calculation. In other cases, the user gets an error.

As such, I also had to change the test files so that they contain the temp0 keyword.

In addition, I have created a new test in the INIT directory to test forgetting of the temp0 keyword in input.in when initial velocities are not provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.01%. Comparing base (2572f92) to head (11edbce).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   84.00%   84.01%           
=======================================
  Files          47       47           
  Lines        6633     6637    +4     
  Branches      768      768           
=======================================
+ Hits         5572     5576    +4     
  Misses        865      865           
  Partials      196      196           

Flag	Coverage Δ
unittests	18.28% <0.00%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
src/init.F90	82.73% <100.00%> (+0.10%)	⬆️

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[danielhollas]

Thanks for the PR and sorry for the delay in review. I will get to it next week, but on a quick skim this looks great!