Adding more docstrings, high level interface improvements

[thomvet]

This PR addresses the following:

• adds a basic readme file
• adds basic examples to documentation
• further improves high level interface (everything starting with JUA...) - now approaching a usable state.
• introduces many tests regarding the above interface.
• increases flexibility in terms of what numerical types are usable directly in nodes; now also includes ComplexF32 and ComplexF64 and Strings (and arrays thereof).
• introduced appropriate testing for the numerical types.
• Complete server.jl docstrings
• Complete highlevel_server.jl docstrings

Client side is still less documented, but I think this could now be merged.

[codecov]

Codecov Report

Attention: Patch coverage is 86.55462% with 48 lines in your changes are missing coverage. Please review.

Project coverage is 41.77%. Comparing base (04af27b) to head (a9b34b2).

Files	Patch %	Lines
src/highlevel_types.jl	83.97%	29 Missing ⚠️
src/highlevel_server.jl	85.10%	7 Missing ⚠️
src/client.jl	50.00%	4 Missing ⚠️
src/highlevel_client.jl	88.88%	4 Missing ⚠️
src/helper_functions.jl	91.30%	2 Missing ⚠️
src/types.jl	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #18      +/-   ##
==========================================
+ Coverage   38.45%   41.77%   +3.31%     
==========================================
  Files          10       12       +2     
  Lines        3573     3799     +226     
==========================================
+ Hits         1374     1587     +213     
- Misses       2199     2212      +13     

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[thomvet]

@martinkosch: Can you have a look and approve/merge if you think it's fine?