modify CSM merge for non datetime subsytems

[CagtayFabry]

Changes

• allow time dependent subsystems that don't use absolute times to be merged into CSM instances with a set reference time
• make sure csm.time_union() returns correct index type without explicit reference time

Related Issues

Closes # (add issue numbers)

Checks

• updated CHANGELOG.md
• updated tests
• updated doc/
• update example/tutorial notebooks

[codecov]

Codecov Report

Merging #268 (045047b) into master (ac9573d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #268   +/-   ##
=======================================
  Coverage   99.65%   99.65%           
=======================================
  Files          75       75           
  Lines        4019     4022    +3     
=======================================
+ Hits         4005     4008    +3     
  Misses         14       14           

Impacted Files	Coverage Δ
weldx/transformations/cs_manager.py	98.61% <100.00%> (+<0.01%)	⬆️

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[CagtayFabry]

@vhirtham the problem is caused when csm.uses_absolute_times is true (there is some DateTimeIndex involved) but csm.reference_time is false (there is no reference time set on a CSM level)

I think this can only happen with a single reference_time LCS involved but not sure about the checks

This fix will return a valid pd.DateTimeIndex in these cases

[CagtayFabry]

@vhirtham changed error behavior of csm.merge() as discussed