improving check for lazyarray + other improvements

[tlienart]

• added a is_type method which offers a robust way of checking if an object is of a given type even if it's a type from a package not loaded in the current environment
• this allows for a more robust check in the LazyArrays saga than checking the propertynames which was arguably a bit crappy
• added a coerce! for dataframes, I figured that it didn't really make sense for arrays as an array is of one time and modfiying it in place to change the type is not a good idea...
• a few modifications to how coerce and _coerce_col worked together to avoid multiple computations of the same objects
• fixed an issue where kwargs were not forwarded properly in coerce(X, ::Dict)
• fixed some tests
• added some more tests for coerce! and the LazyArrays thing (implicitly this tests the is_type function as well)

closes #50 (it may be re-opened in the future for specific formats)

[codecov-io]

Codecov Report

Merging #56 into dev will increase coverage by 1.75%. The diff coverage is 90.54%.

@@            Coverage Diff             @@
##              dev      #56      +/-   ##
==========================================
+ Coverage   90.16%   91.91%   +1.75%     
==========================================
  Files           6        8       +2     
  Lines         183      198      +15     
==========================================
+ Hits          165      182      +17     
+ Misses         18       16       -2

Impacted Files	Coverage Δ
src/ScientificTypes.jl	100% <100%> (+14.28%)	:arrow_up:
src/scitype.jl	68.75% <68.75%> (ø)
src/coerce.jl	95.45% <95.45%> (ø)
src/schema.jl	96.96% <96.96%> (ø)

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