Warn on flattening X

[ivirshup]

In response to #171 (comment)

Throw a warning if X is flattened prior to returning, since that behavior will change in v0.7.

[codecov]

Codecov Report

Merging #172 into master will increase coverage by 0.02%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   78.82%   78.85%   +0.02%     
==========================================
  Files          12       12              
  Lines        2102     2109       +7     
==========================================
+ Hits         1657     1663       +6     
- Misses        445      446       +1

Impacted Files	Coverage Δ
anndata/compat.py	29.62% <100%> (+8.79%)	⬆️
anndata/core/anndata.py	79.38% <80%> (-0.02%)	⬇️

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[falexwolf]

Perfect! 😄