Updating documentation

[AnHeuermann]

Some improvements for the documentation.

Docstrings are now helpfull without the online documentation:

Old:

help?> OMJulia.showQuantities
  standard getXXX() API function same as getQuantities(), but returns all the variables as table

New:

help?> OMJulia.showQuantities
  showQuantities(omc, name=nothing)

  Return DataFrame of all variables parsed from xml file.

  Arguments
  ===========

    •  omc::OMCSession: OpenModelica compiler session.

    •  name::Union{<:AbstractString, Array{<:AbstractString,1}, Nothing}: Name(s) of variables to read from xml file. If nothing is provided read all variables.

  See also getQuantities.

Related Issues

Fixes #87, fixes #85.

Changes

• Adding types to function interfaces
• Adding types to structs
• Refactoring OMCSession into multiple structs
• Document get and set methods
• Simplifying if statements, removing brackets
• Changing prints to @error or @info

[AnHeuermann]

@arun3688 I made a lot of changes to all the interfaces. Can you have a short look at it? I hope the CI will catch most errors, but we don't test the API functions very well.