Release v0.7.0.dev3

Minor updates to the v0.7.0 pre-release, mostly to address documentation issues.

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|new| mkdocs_tabbed option when writing math documentation to file (calliope.Model.math_documentation.write(...)) which will add YAML snippets to all rendered math as a separate "tab" if writing to Markdown.
Requires the PyMdown tabbed extension to render the tabs correctly in an MkDocs project.

|new| List of pre-defined parameters given in the pre-defined math documentation, with references back to the constraints/variables/global expressions in which they are defined (either in the expression string or the where string).

|new| Units and default values for variables and global expressions added to the math documentation.

|new| Variables and global expressions can have a default value, which is used to fill missing array elements when doing math operations.
These default values ensure that NaN doesn't creep into the built optimisation problem math and are set to values that lead to them having no impact on the optimal solution.

|new| Utility function calliope.util.schema.update_model_schema(...) to add user-defined parameters to the model schema / update existing parameters using YAML schema syntax.
calliope.util.schema.reset() can be used to clean the model schema and return to the original, pre-defined schema.

|fixed| Timeseries clustering file can be a non-ISO standard date format.
Both the index and the values of the timeseries (both being date strings) should be in the user-defined config.init.time_format.

|fixed| the decision variable purchased_units is linked to flow_cap even if neither of the parameters flow_cap_min or flow_cap_max have been defined by the user.

|changed| inbuilt math -> pre-defined math and custom math -> pre-defined math in the documentation.