Cleanup the FieldVector<K,1>

Simon Praetoriusrequested to merge
issue/fieldvector-1-constructors into master

Summary

This MR simplified/cleansup/improves the FieldVector<K,1> without reducing its functionality.

There are several issues this MR addresses, especially with the constructors from scalar and from DenseVector due to the complicated enable_if constraints. See also !1040 (comment 85553)

Edited by Simon Praetorius

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