Additional register type conversion rule

[pavel-kirienko]

Allow register.Value.assign() to automatically extend/truncate arrays to avoid issues like https://forum.uavcan.org/t/control-and-debug-kotleta20-esc-with-yakut/1176/3\?u\=pavel.kirienko

[coveralls]

Pull Request Test Coverage Report for Build 38519968

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.01%) to 95.684%

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Files with Coverage Reduction	New Missed Lines	%
/home/appveyor/projects/pyuavcan/pyuavcan/application/plug_and_play.py	1	92.24%

Totals
Change from base Build 38321402:	-0.01%
Covered Lines:	13406
Relevant Lines:	13852

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💛 - Coveralls