LPF: Throw if calling udpate unconfigured

[christophfroehlich]

@saikishor raised a good point here: #222 (comment)

And I removed the shadowed class variables (#219 fails), I don't see the point in saving them anyways.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 51.20%. Comparing base (7d008a4) to head (646838f).

Files with missing lines	Patch %	Lines
include/control_toolbox/low_pass_filter.hpp	33.33%	2 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff               @@
##           ros2-master     #229      +/-   ##
===============================================
+ Coverage        51.17%   51.20%   +0.03%     
===============================================
  Files               10       10              
  Lines              510      496      -14     
  Branches            64       59       -5     
===============================================
- Hits               261      254       -7     
+ Misses             221      215       -6     
+ Partials            28       27       -1     

Flag	Coverage Δ
unittests	51.20% <50.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
include/control_filters/low_pass_filter.hpp	72.72% <100.00%> (+12.72%)	⬆️
include/control_toolbox/low_pass_filter.hpp	76.19% <33.33%> (-3.41%)	⬇️

[saikishor]

Thank you!