Splines and plot_erp

[vladdez]

Fixed:

• gap for legend in plot_erp #231
• categorical_group = false causes error due to AoG #235
• plot_erp: layout legend_position not working #225 - this parameter was deprecated at all
• plot_erp legend problem #234
• remove legend.tellwidth = false from the plotconfgs - better layout
• Bug: errorribbon not dislayed correctly #55 and Extra standart ribbon in plot_erp #238
• Colorful terminal output of unfold is not shown properly in docs #216
• No warning that variable for col parameter in mapping has to be nonnumeric. #56
• Allow AoG.sorter( ) #232
• config_kwargs!(config) - what does it do? #172

Spline plot

• Non-Linear Plot #36 - here we added a new type of plot plot_splines
• knots with dashed line
• subplots for each spline term
• documentation page and config

[codecov]

Codecov Report

Attention: Patch coverage is 95.34884% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.59%. Comparing base (0c5d556) to head (01e8c1c).
Report is 112 commits behind head on main.

Files with missing lines	Patch %	Lines
src/plot_erp.jl	93.33%	1 Missing ⚠️
src/plot_splines.jl	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #237      +/-   ##
==========================================
+ Coverage   87.63%   88.59%   +0.96%     
==========================================
  Files          18       20       +2     
  Lines         849      938      +89     
==========================================
+ Hits          744      831      +87     
- Misses        105      107       +2     

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