Add pre-calc hydraulic module

[joroeder]

This PR adds a module with functions for the for pre-calculation of hydraulic parameters for the district heating network optimisation.

• [x] Review formulas
• [x] Add/fix docstrings (in numpy format)
• [x] Fix code style (flake8)
• [x] Add examples of pre calculation procedure
• [x] Add tests
  • [x] Add test for different flow regimes of calculation of p
• [x] Update whatsnew / contributors / etc.

[coveralls]

Pull Request Test Coverage Report for Build 2333061720

• 114 of 118 (96.61%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.2%) to 56.495%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
dhnx/optimization/precalc_hydraulic.py	112	116	96.55%
<!--	Total:	114	118	96.61%	-->

Totals
Change from base Build 2332306241:	2.2%
Covered Lines:	1144
Relevant Lines:	1916

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💛 - Coveralls

[joroeder]

The new module of this PR requires CoolProp. However, CoolProp does not work at the moment with python 3.9+. When the next release of CoolProp is out, this should be fixed (https://github.com/CoolProp/CoolProp/pull/2097), and python 3.9 can be added to the CI again.