Add helpful message when ProductManifold is used without RAT.jl

[mateuszbaran]

I had to change representation_size for ProductManifold for this but I think it doesn't make that much sense anyway.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.90%. Comparing base (8469ef9) to head (2753277).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #201   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files          31       31           
  Lines        3294     3300    +6     
=======================================
+ Hits         3291     3297    +6     
  Misses          3        3           

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[kellertuer]

One could consider this breaking since it introduces an error in some cases; but I think instead of nothing being more careful with that is more like improve the user experience, so that is fine with me even in a nonbreaking sense.

[mateuszbaran]

Yes, it's technically breaking but it's a part that no one should rely on.