Simplifications with the IIASA API module

[danielhuppmann]

Description of PR

While working on the IIASA-API implementation, I noticed a few clean-up steps that make the code slightly more concise.

• Reusable headers as a property
• Clear separation between the url of the services-manager versus the database-API

There are no functional changes in this PR.

[codecov[bot]]

Codecov Report

Merging #683 (6029ce2) into main (2228359) will increase coverage by 0.0%. The diff coverage is 84.3%.

@@          Coverage Diff          @@
##            main    #683   +/-   ##
=====================================
  Coverage   94.7%   94.7%           
=====================================
  Files         59      59           
  Lines       5812    5807    -5     
=====================================
- Hits        5506    5504    -2     
+ Misses       306     303    -3     

Impacted Files	Coverage Δ
pyam/iiasa.py	86.7% <84.3%> (+0.9%)	:arrow_up:

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