Add api for tpdo and rpdo

[khauersp]

I was trying to use a local node and interacting with the setup for the tpdo and rpdo objects wasn't the most clear on how to set them up and get them working properly so I added some more functions to help with that process.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 44.44444% with 15 lines in your changes missing coverage. Please review.

Project coverage is 71.45%. Comparing base (ffbd10f) to head (ca6347c).

Files with missing lines	Patch %	Lines
canopen/pdo/base.py	50.00%	3 Missing and 3 partials ⚠️
canopen/node/local.py	28.57%	5 Missing ⚠️
canopen/pdo/__init__.py	33.33%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #545      +/-   ##
==========================================
+ Coverage   71.36%   71.45%   +0.08%     
==========================================
  Files          26       26              
  Lines        3129     3156      +27     
  Branches      480      487       +7     
==========================================
+ Hits         2233     2255      +22     
- Misses        765      767       +2     
- Partials      131      134       +3     

Files with missing lines	Coverage Δ
canopen/network.py	90.05% <100.00%> (+0.11%)	⬆️
canopen/pdo/__init__.py	71.42% <33.33%> (-6.35%)	⬇️
canopen/node/local.py	81.11% <28.57%> (-4.44%)	⬇️
canopen/pdo/base.py	67.35% <50.00%> (+2.11%)	⬆️