Improve stalagmite parameters and performance

[IntegratedQuantum]

Slope is now configurable with the slope parameter, default is 0.25. Furthermore I added a pointiness parameter which makes the stalagmite a bit more pointy at the top, while creating a more curved bottom.

I also decided to change the model a bit: Instead of turning into a sphere at the bottom, the stalagmites are now dual-sided:

@careeoki please tell me what you think about this and if there are any more things you need for #2464.

fixes #2501
also greatly improves performance (~15×) compared to the previous generator.

[codemob-dev]

We should make other speleothem as well https://en.wikipedia.org/wiki/Speleothem

[careeoki]

Yes, that looks great, thank you!

[IntegratedQuantum]

I wasn't happy with the pointiness parameter, so I decided to change the model a bit: There are now two parameters: baseSlope (defaults to 4) and topSlope (defaults to baseSlope) which parametrize the slop at the top and the base, this should give you more intuitive control over the shape.
Here some examples:
default (baseSlope = topSlope = 4):

pointier at the top (baseSlope = 4, topSlope = 8):

flat base (baseSlope = 0, topSlope = 8):

parabola hill (baseSlope = 8, topSlope = 0):

[careeoki]

That all looks very useful.