refactor(pakbase): standardize ipakcb docstrings/defaults

[wpbonelli]

• close Default budget file unit number (ipakcb) #803 per recommended approach
• add Package.set_cbc_output_file method, use from flopy.modflow packages
• make ipakcb defaults consistent, ipakcb=None sets 0 and no output file, "default" sets 53
• make ipakcb docstrings consistent

questions

• warn about uniqueness requirement? If so where?

[codecov]

Codecov Report

Merging #2001 (c01c305) into develop (121ca78) will decrease coverage by 0.1%.
The diff coverage is 96.2%.

Additional details and impacted files

@@            Coverage Diff            @@
##           develop   #2001     +/-   ##
=========================================
- Coverage     72.8%   72.8%   -0.1%     
=========================================
  Files          259     259             
  Lines        59111   59050     -61     
=========================================
- Hits         43066   42990     -76     
- Misses       16045   16060     +15     

Files	Coverage Δ
flopy/mfusg/mfusgbcf.py	80.4% <ø> (ø)
flopy/mfusg/mfusglpf.py	80.4% <ø> (ø)
flopy/mfusg/mfusgwel.py	82.5% <ø> (-1.6%)	⬇️
flopy/modflow/mfbcf.py	89.1% <100.0%> (+0.3%)	⬆️
flopy/modflow/mfdrn.py	83.5% <100.0%> (-0.7%)	⬇️
flopy/modflow/mfdrt.py	67.6% <100.0%> (+<0.1%)	⬆️
flopy/modflow/mfevt.py	94.9% <100.0%> (-0.1%)	⬇️
flopy/modflow/mffhb.py	63.3% <100.0%> (-0.1%)	⬇️
flopy/modflow/mfghb.py	89.8% <100.0%> (-0.5%)	⬇️
flopy/modflow/mflak.py	78.4% <100.0%> (+<0.1%)	⬆️
... and 14 more

... and 5 files with indirect coverage changes