PR for #67: X-axis as category with string values

[hcpchris]

Fixed #67 . Now when .from_array() is called to construct a data point and:

• the array does not explicitly indicate property values, AND
• the x value in the array is a string
  the x value is also propagated into the data point's name property.

[codecov]

Codecov Report

Merging #68 (d1010ad) into develop (88a802b) will decrease coverage by 0.03%.
The diff coverage is 8.33%.

@@             Coverage Diff             @@
##           develop      #68      +/-   ##
===========================================
- Coverage    90.31%   90.28%   -0.03%     
===========================================
  Files          206      206              
  Lines        27495    27507      +12     
  Branches      1786     1792       +6     
===========================================
+ Hits         24831    24834       +3     
- Misses        1974     1982       +8     
- Partials       690      691       +1     

Files Changed	Coverage Δ
highcharts_core/options/series/data/bar.py	80.43% <0.00%> (-0.71%)	⬇️
highcharts_core/options/series/data/boxplot.py	78.72% <0.00%> (-1.14%)	⬇️
highcharts_core/options/series/data/bullet.py	64.70% <0.00%> (-1.97%)	⬇️
highcharts_core/options/series/data/range.py	80.00% <0.00%> (-0.99%)	⬇️
highcharts_core/options/series/data/cartesian.py	72.32% <25.00%> (+0.04%)	⬆️