Modify files regarding lead-field matrix

[Wirkungstreffer]

Modified example lead-field matrix notebook and lead-field function script to fix some Python version incompatibility, also adding some functions like generating simulated source activities, calculating, plotting, and storing simulated EEG signals, and so on.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 64 lines in your changes are missing coverage. Please review.

Project coverage is 86.92%. Comparing base (6be0d37) to head (ffdac07).

Files	Patch %	Lines
neurolib/utils/leadfield.py	0.00%	64 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
- Coverage   87.91%   86.92%   -0.99%     
==========================================
  Files          66       66              
  Lines        5351     5414      +63     
==========================================
+ Hits         4704     4706       +2     
- Misses        647      708      +61     

Flag	Coverage Δ
unittests	86.92% <0.00%> (-0.99%)	:arrow_down:

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