Fix error during assembly in Form for different mesh- and field-dimensions

[adtzlr]

fixes #674

Also changed within this PR

• Don't require a bilinearform in FormItem(bilinearform=None). An empty FormItem is now a valid item in a Step. For empty vectors/matrices, the shape is inferred from sum(FieldContainer.fieldsizes) instead of FieldContainer.fields[0].values.size. closes FormItem: Don't require a bilinearform for a constant linearform #676 and closes FormItem: Infer vector/matrix shape from sum(FieldContainer.fieldsizes) #677

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.88%. Comparing base (8243d74) to head (3001917).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #675      +/-   ##
==========================================
+ Coverage   96.85%   96.88%   +0.02%     
==========================================
  Files          94       94              
  Lines        4963     4970       +7     
==========================================
+ Hits         4807     4815       +8     
+ Misses        156      155       -1     

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