Fix #6: let show_code display/print code

- `get_code` will keep the previous behavior of `show_code`
- `get_code_str` will be equivalent to previous `str(show_code(...))`
10 jobs for show_code in 7 minutes and 44 seconds (queued for 2 seconds)
Name Stage Failure
tests-and-coverage Test
14.23s call     pystencils_tests/
13.64s call pystencils_tests/
12.42s call pystencils_tests/test_small_block_benchmark.ipynb::test_small_block_benchmark.ipynb
= 5 failed, 279 passed, 8 skipped, 1 xfailed, 1 xpassed, 345 warnings in 155.10s (0:02:35) =
Uploading artifacts...
coverage_report: found 78 matching files
test-report: found 4 matching files

Uploading artifacts to coordinator... ok
id=341087 responseStatus=201 Created token=68A6eVWd
ERROR: Job failed: exit code 1