Add BoxGirderSection class; extend line widths

Author: robbievanleeuwen

.travis.yml

@@ -5,6 +5,7 @@ before_install:
 
 python:
   - "3.6"
+  - "3.7"
 
 install:
   - python setup.py install

README_pypi.rst

@@ -5,11 +5,10 @@
 
 |Build Status| |Documentation Status|
 
-A python package for the analysis of arbitrary cross-sections using the
-finite element method written by Robbie van Leeuwen. *sectionproperties*
-can be used to determine section properties to be used in structural
-design and visualise cross-sectional stresses resulting from
-combinations of applied forces and bending moments.
+A python package for the analysis of arbitrary cross-sections using the finite element method
+written by Robbie van Leeuwen. *sectionproperties* can be used to determine section properties to
+be used in structural design and visualise cross-sectional stresses resulting from combinations of
+applied forces and bending moments.
 
 `Subscribe`_ to the mailing list!
 
@@ -47,10 +46,9 @@ Once *meshpy* has been installed, *sectionproperties* can be installed:
 Documentation:
 --------------
 
-*sectionproperties* has a fully documented python API which you can find
-at https://sectionproperties.readthedocs.io/. To read more about the
-theory behind the program, its implementation and some more examples,
-check out my blog at https://robbievanleeuwen.github.io/.
+*sectionproperties* has a fully documented python API which you can find at
+https://sectionproperties.readthedocs.io/. To read more about the theory behind the program, its
+implementation and some more examples, check out my blog at https://robbievanleeuwen.github.io/.
 
 Current Capabilities:
 ---------------------