[MRG] Fix error when --scaled 1 is set.

[ctb]

Fixes #152 by setting max_hash to be 0 when --scaled=1.

Also addresses #112 by adding MINHASH_MAX_HASH and a getter to the extension module,
and adds a module-level getter for DEFAULT_SEED (#96).

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #156 into master will decrease coverage by 0.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
- Coverage   85.55%   85.34%   -0.22%     
==========================================
  Files          13       13              
  Lines        1828     1829       +1     
  Branches       52       52              
==========================================
- Hits         1564     1561       -3     
- Misses        255      259       +4     
  Partials        9        9

Impacted Files	Coverage Δ
sourmash_lib/commands.py	89.13% <100%> (-0.64%)	⬇️
sourmash_lib/__init__.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2dcedd7...b2e26ee. Read the comment docs.

[ctb]

Ready for review!

[luizirber]

LGTM