Further work on documentation

Further work on documentation:

* Early work on documentation of core FSIC script (`fsic.py`)
* Minor edits to `solve.rst`
* Early work on `specify.rst`, to present Godley and Lavoie (2007) SIM
  model
Add description of market-clearing in Model SIM

Add description of market-clearing in Godley and Lavoie (2007) Model SIM
to Sphinx documentation.

[ci skip]
Add file to document installation procedure

Add file to Sphinx documentation to describe installation procedure.
Tidy up transactions matrix and add more on SIM

Tidy up transactions matrix in Sphinx documentation (to be LaTeX -> PDF
friendly).

Add sections to documentation on disposable income and household
consumption.

[ci skip]
Impose 64-bit floats on file reading

Impose 64-bit floats on calls to `pandas.read_csv()`.
Early work on Model class tests

Early work on Model class test to check data-reading code.
Exclude `doc/build` from `MANIFEST.in`

Exclude `doc/build` from `MANIFEST.in`, to exclude stray
makefiles (e.g. for LaTeX conversion).
Initial implementation of `Model.read_data()`

Initial (rough) implementation of `Model.read_data()`, to deal with simplest
case of a single data file and no currently-stored data.
Add NumPy numeric type to `read_csv()` call

Add `dtype` argument to pandas `read_csv()` call in Model class test.
Centralise preferred numeric type

Centralise preferred numeric type for NumPy and pandas objects into new,
automatically-generated, `settings.py` file (written by `setup.py`).
Add indexing to read code

Add indexing to `io` module code, for compatibility with new `Model`
read code.

Update test input data accordingly.

Generalise `Model` read code to match treatment in `io.readers`.