feat(binaryfile): add reverse() methods

[wpbonelli]

Add reverse() methods to CellBudgetFile and HeadFile, with corresponding tests. Originally planned as part of #1753 but expediting separately since this will be useful to develop PRT tests and example notebooks.

• CellBudgetFile.reverse()
• HeadFile.reverse()

To use either method, a tdis kwarg must have been provided in the binary file object constructor.

Also reorganize/expand some docstrings in binaryfile.py, add a get_nrecords() method to LayerFile, and split tests into test_cellbudgetfile.py and test_binaryfile.py

[codecov]

Codecov Report

Merging #1829 (6e41428) into develop (8c330a7) will increase coverage by 0.0%.
The diff coverage is 87.8%.

@@           Coverage Diff           @@
##           develop   #1829   +/-   ##
=======================================
  Coverage     72.2%   72.2%           
=======================================
  Files          257     257           
  Lines        56414   56508   +94     
=======================================
+ Hits         40751   40835   +84     
- Misses       15663   15673   +10     

Files Changed	Coverage Δ
flopy/utils/datafile.py	74.3% <83.3%> (+0.2%)	⬆️
flopy/utils/binaryfile.py	80.8% <88.0%> (+0.7%)	⬆️

... and 2 files with indirect coverage changes