Added import Base: isassigned so that view works correctly on QuasiAr…

[macd]

Currently, the following fails with a missing method

    using QuasiArrays
    A = QuasiArray(randn(5,2), (0:0.5:2, [1,4]))
    V = view(A, 0.0, [1,4])

This PR just adds that import to QuasiArrays.jl. There is a test for this in test_quasipermutedims and it does pass, but fails
outside of the test environment.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (b0741da) to head (9e691cc).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #116       +/-   ##
==========================================
- Coverage   64.25%   0.00%   -64.26%     
==========================================
  Files          23      23               
  Lines        1835    1818       -17     
==========================================
- Hits         1179       0     -1179     
- Misses        656    1818     +1162     

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