Fixes for subgraph/GeoLocation/MetaGraph typing

[bjmommers]

• Graph object typing in add_graph! now depends on the OSMGraph types. This ensures subgraphs are generated with graph objects that match the typing of the parent OSMGraph.graph
• GeoLocation constructor methods added to capture Real inputs and convert to float. This allows GeoLocations to still be correctly produced when Dict parsing of coordinates incorrectly uses an integer type.
• Other minor fixes found during testing of the above: methods used to generate a MetaGraph in add_graph! were not imported; and the transpose of the SparseMatrix of weights needed to be instantiated (done via copy) for passing into findnz during MetaGraph construction.

[codecov-commenter]

Codecov Report

Merging #72 (b8de26e) into master (7d1c144) will increase coverage by 1.33%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #72      +/-   ##
==========================================
+ Coverage   77.15%   78.49%   +1.33%     
==========================================
  Files          13       13              
  Lines        1007     1009       +2     
==========================================
+ Hits          777      792      +15     
+ Misses        230      217      -13     

Impacted Files	Coverage Δ
src/graph.jl	87.83% <100.00%> (+6.87%)	:arrow_up:
src/types.jl	80.76% <100.00%> (+1.60%)	:arrow_up:

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