feat: added option to run isothermally pipe Beggs and Brills

[Sviatose]

1. Made the variables private to ensure that their internal state is not directly accessible or modifiable by other classes.
2. Added option to run isothermally
3. Added more get methods (angle, elevation)
4. fix: return total length instead of segment length

[EvenSol]

I see the test of beggs and brills fail with a null pointer exception. I can look into that later. Any idea why it happens ? @Sviatose

[Sviatose]

Hm, does not fail when I run test locally, but it was a problem with PH flash