Create content explaining the Analyser class

[kerimoyle]

It would be good to have an explanation of how/why the Analyser class works so that this can be added to the User Guides documentation. Three things to do:

1. Flesh out the How to > Analyse a Model page:
   https://staging.libcellml.org/documentation/guides/v0.1.0/howto/analyse_model
   Once this is added, make sure you add the file into the index at:
   https://staging.libcellml.org/documentation/guides/v0.1.0/howto/index
   (See an example of actions documentation, this one for the Annotator: https://staging.libcellml.org/documentation/guides/v0.1.0/howto/annotate_model )

2. Add information to the General Info > Analyser page:
   https://staging.libcellml.org/documentation/guides/v0.1.0/asides/analyser

3. Fill in the error codes about the Analyser class:
   http://staging.libcellml.org/documentation/guides/v0.1.0/runtime_codes/index