Multiple dispatch for viscous displacement solver

[JanJereczek]

For now, updating the (vertical) viscous displacement relies on if-else statements that select the right method to call depending on the user's choice. This is not particularly elegant, since unused parameters need to be defined. Most importantly, it does not allow the user to easily extend the code base with a new updating method. The new structure should look like:

abstract type ElasticLithoTransientAsteno end

struct LVELVA <: ElasticLithoTransientAsteno
....
end

method = LVELVA(params...)

function update_viscous_displacement(args...)
...
end