Clean-up exposing internal methods and attributes

[danielhuppmann]

Description of PR

As part of preparing for release 2.0, this PR does some housekeeping and reduces the all-over-the-place exposure of pyam-internals.

[codecov[bot]]

Codecov Report

Merging #764 (cd4e8bf) into main (1044c28) will decrease coverage by 0.1%. The diff coverage is 97.0%.

@@           Coverage Diff           @@
##            main    #764     +/-   ##
=======================================
- Coverage   94.4%   94.4%   -0.1%     
=======================================
  Files         59      61      +2     
  Lines       6086    6121     +35     
=======================================
+ Hits        5750    5780     +30     
- Misses       336     341      +5     

Files Changed	Coverage Δ
pyam/unfccc.py	26.0% <66.6%> (+1.5%)	:arrow_up:
pyam/utils.py	90.5% <66.6%> (-2.3%)	:arrow_down:
pyam/__init__.py	68.7% <100.0%> (-1.9%)	:arrow_down:
pyam/aggregation.py	99.1% <100.0%> (+<0.1%)	:arrow_up:
pyam/core.py	95.2% <100.0%> (+<0.1%)	:arrow_up:
pyam/iiasa.py	87.1% <100.0%> (+<0.1%)	:arrow_up:
pyam/plotting.py	92.9% <100.0%> (+<0.1%)	:arrow_up:
pyam/run_control.py	72.0% <100.0%> (ø)
pyam/statistics.py	85.6% <100.0%> (+<0.1%)	:arrow_up:
pyam/str.py	100.0% <100.0%> (ø)
... and 20 more