Improve checks on 'sde()' method for user defined 'SDE' or 'SDEs' subclasses

[m-cip]

• The type, length and keys of sde() returned values are now checked and exceptions raised, hinting at the cause of the problem.
• Equations may omit zero terms, e.g. dX = sigma*X*dW may be stated as {'dw': X} as well as {'dw': X, 'dt': 0}.

The new checks insist on the inner python loop (the true inner loop runs across paths and is delegated to numpy), with no recognizable impact on performance.

Fixes #20

[codecov-io]

Codecov Report

Merging #21 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #21      +/-   ##
==========================================
+ Coverage   91.63%   91.70%   +0.06%     
==========================================
  Files           7        7              
  Lines        1889     1904      +15     
  Branches      272      277       +5     
==========================================
+ Hits         1731     1746      +15     
  Misses         96       96              
  Partials       62       62              

Impacted Files	Coverage Δ
sdepy/integration.py	91.86% <100.00%> (+0.24%)	⬆️

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