Custom user normalization & verbose flag for LICs

[Adnan-Ali-Ahmad]

• Added verbose flag to LIC wrapper
• Added string check when plot::parse_layer parses the "norm" keyword, so that users can provide their own custom normalization instead of being constrained to the ones in get_norm.
• Added "SymLog" to get_norm, which plots simultaneously negative & positive in logarithmic scaling (eg. for radial velocities, or B_field components). Below is an example of this:
  

[nvaytet]

Some small comments.

Can you change the PR title to something that describes the changes a little better, then I can use this when making a release, as github will generate release notes automatically from PR titles ;-)