Developed model to work with templates
Template and python structure development

Continuing to develop templates for writing GAMS code and the python structures for controlling a model and constructing elements.
Support for n-dim subsets and bugfixes

Bugfixes in templates (subsets could be written before supersets) and adding support for multi-dimensional subsets.
Added functions to create parameters from pandas types
Development of templates and small fixes in model and types
Added upper/lower limits and possibilities to fix values for variables.
Cleaned up gams functions and added possibility for conditionals
Further development after testing with WEDD2
Moved add/subtract to separate class to be able to implement that for sets.
Fixed bug in index.
Make it possible to use another template
Added blocks to template, added possibility of choosing template.
Added block for output
Merge branch 'develop' of github.com:joelgoop/gamspy into develop