fix(str and repr): better repr and str output for transient data with multiple blocks (#2058)

[scottrp]

When transient data contains multiple blocks of data this fix:

1. Makes sure that the first block of data's str/repr gets called (it use to be the last "active" block got displayed)
2. Flopy use to just display just one block of data. Now, if there are additional blocks, a message is added to the end of the str/repr indicating that more data is present.

Also added text for external data indicating that external is not displayed in the repr (it was not displayed before but no text indicated this)

For example, getting the str of ghb obs continuous data:

str(model.get_package("ghb_obs").continuous)

Might produce the following output:

{open/close AdvGW_tidal.ghb.obs_continuous_ghb_flows.csv.txt}
(External data not displayed)
...
and 1 additional data blocks

[codecov]

Codecov Report

Attention: 57 lines in your changes are missing coverage. Please review.

Comparison is base (626563a) 73.0% compared to head (cb1e5e4) 73.0%.
Report is 6 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #2102   +/-   ##
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  Coverage     73.0%   73.0%           
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  Files          259     259           
  Lines        59415   59491   +76     
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+ Hits         43401   43457   +56     
- Misses       16014   16034   +20     

Files	Coverage Δ
flopy/mf6/data/mfdataarray.py	60.9% <50.0%> (-0.1%)	⬇️
flopy/mf6/data/mfdatalist.py	64.9% <50.0%> (+0.5%)	⬆️
flopy/mf6/data/mfdatascalar.py	60.0% <50.0%> (-0.6%)	⬇️
flopy/mf6/data/mfdatastorage.py	71.2% <0.0%> (-0.3%)	⬇️
flopy/mf6/data/mfdata.py	72.9% <6.6%> (-2.9%)	⬇️
flopy/mf6/data/mfdataplist.py	74.3% <44.2%> (-1.9%)	⬇️

... and 14 files with indirect coverage changes

[langevin-usgs]

Would it make sense to paste some examples into this PR for what the output will look like for str and repr?

[langevin-usgs]

Hey @scottrp, this seems reasonable to me. Let's merge it in.