Initial work on build chain

New build() function in Build class.

Update fsic-build.py to call incomplete build() function.
New parse_chunks() function as part of rewrite

New parse_chunks() function to process Markdown code chunks.

Move code-generation statements from fsic-build.py to Build class.
Slight update to parse_chunks()

Update to parse_chunks() to use `language` instead of `type` as a
keyword argument.

Add `else` clause to trap unrecognised `language` arguments.
Early work on configuration-file string parsing

Early work on Build-class parse_chunks() function to handle INI-style
code blocks.
New utilities subpackage

New folder and __init__.py files for utilities and utilities.tests
subpackages.
New utilities.string module

New string module for utilities subpackage.

New file to store string tests.
Initial commit of indent() function

First (rough) draft of indent() function for utilities.string
subpackage. Further work required.
Rework string indent function

Rename indent function to indent_lines() and update keyword-argument
names.

Update test cases accordingly.
Early work on model solve stack

New placeholder for solve_equations() code.

Update to Build class to insert code after call to indent_lines().

Add incomplete solve_period() function to Model class.