Fix ambiguity error in similar with 3 or more blocked axes

[mtfishman]

I found that similar calls such as:

using BlockArrays
r = blockedrange([2, 2])
similar(Array{Float64}, (r, r, r))

(with inputs of 3 or more blocked axes) lead to ambiguity errors between the StridedArray and AbstractArray versions which output BlockedArray and BlockArray respectively, this PR fixes that ambiguity by defining a more specialized similar method only for AbstractBlockedUnitRange inputs. The existing definitions accept mixtures of blocked, non-blocked, and integer dimensions which is more prone to ambiguities.

Related to #196 and #489.

I also noticed these definitions are not so friendly for GPU array types but that is a separate matter...

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.37%. Comparing base (7ebbea7) to head (9448261).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #494      +/-   ##
==========================================
- Coverage   94.52%   94.37%   -0.16%     
==========================================
  Files          19       19              
  Lines        1789     1812      +23     
==========================================
+ Hits         1691     1710      +19     
- Misses         98      102       +4     

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