Implement review suggestions

[trichter]

This PR includes the following changes:

• add note about numpy flag, closes [JOSS review] Errors when install toeplitz #18
• mention the RTZ coordinate system in the method section and in the paper, closes [JOSS review] Mention the RTZ coordinate system in the method section #19
• add links of tutorial notebooks to the documentation, closes [JOSS review] Add the links of tutorial notebooks to the documentation #20

[codecov-io]

Codecov Report

Merging #22 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #22   +/-   ##
=======================================
  Coverage   90.68%   90.68%           
=======================================
  Files          17       17           
  Lines        1965     1965           
=======================================
  Hits         1782     1782           
  Misses        183      183

Impacted Files	Coverage Δ
rf/__init__.py	85.71% <ø> (ø)	⬆️

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