Update default parameters for the evaluation of g-functions

[MassimoCimmino]

#146 introduced the 'equivalent' solver which significantly speeds up calculations.
#135 introduced non-uniform discretization of finite line sources. This decreases calculation times (due to the lowered required number of segments) and increases accuracy.

Currently, the default solver is 'similarities' and the default discretization is nSegments=12 with a uniform discretization.

This issue is to possibly update the default parameters to use the 'equivalent' solver and a non-uniform discretization. nSegments=8 and the utilities.segment_ratios() function to define the discretization seem to produce good results based on #136.

Default parameters can be revised again after #153.

Required tasks are to update default parameters, examples, and tests.