feat: add an ability to override CoolProp backend when constructing a Fluid instance

[portyanikhin]

Example:

from pyfluids import Fluid, FluidsList

default = Fluid(FluidsList.Water)
assert default.coolprop_backend == "HEOS"

custom = Fluid(FluidsList.Water, coolprop_backend="IF97")
assert custom.coolprop_backend == "IF97"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (49232d3) to head (db95b8c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #90   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         1270      1274    +4     
=========================================
+ Hits          1270      1274    +4     

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