Handle unmapped attributes

[rly]

This PR fixes errors that will occur when hdmf-dev/hdmf#196 is merged.

• Make explicit that the NWBFile/general/intracellular_ephys/filtering dataset does not map to any attribute in the NWBFile class.
• Make explicit that the TimeSeries/sync group does not map to any attribute in the TimeSeries class.
• Fix a test where a DataFrame had the column name "description". Since "description" is an attribute of DynamicTable, the name "description" is not allowed for new column names (see Raise error if expected attribute does not exist on a Container hdmf-dev/hdmf#196)

[codecov]

Codecov Report

Merging #1105 into dev will increase coverage by 0.11%.
The diff coverage is 88.23%.

@@            Coverage Diff            @@
##              dev   #1105      +/-   ##
=========================================
+ Coverage   67.19%   67.3%   +0.11%     
=========================================
  Files          38      38              
  Lines        2274    2288      +14     
  Branches      394     394              
=========================================
+ Hits         1528    1540      +12     
- Misses        677     679       +2     
  Partials       69      69

Impacted Files	Coverage Δ
src/pynwb/io/base.py	25.92% <0%> (-1%)	⬇️
src/pynwb/io/file.py	77.94% <100%> (+2.13%)	⬆️

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