Make an explicit API for new subtypes to work from

[richardreeve]

@claireh93 I've done some fairly minor surgery to the Diversity package to make it easier for you to provide the functions that your Ecosystem is going to need. The functions that need (or may need) overwriting are now all in src/API.jl in the API branch and nearly all start with _. See what you think.

[coveralls]

Coverage decreased (-2.8%) to 96.117% when pulling a73098cd26cacacc046e3c1417e1fd517be261c2 on API into da573c15eaa9f858b5fa94f0873dfb1dc726bc70 on master.

[codecov-io]

Codecov Report

Merging #19 into master will decrease coverage by 2.83%. The diff coverage is 78.84%.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage   98.95%   96.11%   -2.84%     
==========================================
  Files           8        9       +1     
  Lines         286      309      +23     
==========================================
+ Hits          283      297      +14     
- Misses          3       12       +9

Impacted Files	Coverage Δ
src/DiversityMeasure.jl	100% <ø> (ø)	:arrow_up:
src/Interface.jl	75% <75%> (-25%)	:arrow_down:
src/API.jl	79.16% <79.16%> (ø)
src/Metacommunity.jl	96.2% <83.33%> (ø)	:arrow_up:

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[coveralls]

Coverage decreased (-2.8%) to 96.117% when pulling c7815c4904b759d191695946a9fe43de4ccd3371 on API into da573c15eaa9f858b5fa94f0873dfb1dc726bc70 on master.

[coveralls]

Coverage decreased (-2.8%) to 96.117% when pulling 1c758f60d145b9e981a50805b9ae224dd0e1d44a on API into da573c15eaa9f858b5fa94f0873dfb1dc726bc70 on master.