Require get_size() implementation by user

[rgaudin]

getSize being implemented in the wrapper, and its implementation relying on returning the size of get_data(), it prevented the use of get_filename (allowed by libzim).

This now requires user to implement get_size(), not making any assumption about it.

• API break: get_size() now mandatory on Article
• WritingBlob now exposes a size() that returns the C Blob's size so we can still fetch the size from get_data()
• tests checking for implementation of mandatory methods adapted so get_size returns a positive size when testing get_data (otherwise it's skipped)
• Added a test for get_filename() usage

[codecov[bot]]

Codecov Report

Merging #78 into master will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #78   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           99       103    +4     
=========================================
+ Hits            99       103    +4     

Impacted Files	Coverage Δ
libzim/writer.py	100.00% <100.00%> (ø)

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[rgaudin]

rebased