Separate condition deserialization from base Condition via ConditionFactory

[derekpierre]

Type of PR:

• Bugfix
• Feature
• Documentation
• Refactor
• Other

Required reviews:

How many reviews does the PR author need?

• 1
• 2
• 3

What this does:
Some cleanup, post #267

Issues fixed/closed:

• Fixes #...

Why it's needed:

Explain how this PR fits in the greater context of the NuCypher Network.
E.g., if this PR address a nucypher/productdev issue, let reviewers know!

Notes for reviewers:

What should reviewers focus on?
Is there a particular commit/function/section of your PR that requires more attention from reviewers?

Is the use of exported constant types the best way? Static members could be alternatives?

[codecov-commenter]

Codecov Report

Merging #292 (539d5dc) into alpha (67e43cd) will increase coverage by 0.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            alpha     #292      +/-   ##
==========================================
+ Coverage   80.99%   81.25%   +0.25%     
==========================================
  Files          36       36              
  Lines        1021     1035      +14     
  Branches      110      110              
==========================================
+ Hits          827      841      +14     
  Misses        186      186              
  Partials        8        8              

Files Changed	Coverage Δ
src/conditions/base/contract.ts	95.65% <100.00%> (+0.19%)	⬆️
src/conditions/base/index.ts	100.00% <100.00%> (ø)
src/conditions/base/rpc.ts	100.00% <100.00%> (ø)
src/conditions/base/time.ts	100.00% <100.00%> (ø)
src/conditions/compound-condition.ts	100.00% <100.00%> (ø)
src/conditions/condition.ts	97.43% <100.00%> (+0.06%)	⬆️
src/conditions/index.ts	100.00% <100.00%> (ø)
src/conditions/predefined/erc721.ts	100.00% <100.00%> (ø)

[manumonti]

LGTM 👏
I like these changes