#4 Support variable grid spacings

[Andreas Nicolai]

Add option in the project file or command line to enable variable grid spacings to be used during grid generation.

[Andreas Nicolai]

Configure via command line argument:

--variable-disc=<min-dx>:<stretch-factor>

Minimum grid spacing is used as initial guess. Algorithm:

• use bi-tanh stretching function
• start with estimated n until stretch factor is equal or less than given stretch factor

Example:

--variable-disc=0.001;2