Improve code style of inline image code

Please forgive the removal of the line ending ^M below.
I tried for at least half an hour to convince git to ignore this.

Test this by running
`close all; matlab2tikz_acidtest('testsuite', @ACID, 'testFunctionIndices', [114], 'figureVisible', true);`
and inspecting `test114-converted.tex`.

Before:
```
...
\fill [mycolor7] (axis cs:0.5,0.5) rectangle (axis cs:1.5,1.5);\fill [mycolor8] (axis cs:1.5,0.5) rectangle (axis cs:2.5,1.5);\fill [mycolor9] (axis cs:2.5,0.5) rectangle (axis cs:3.5,1.5);\fill [mycolor4] (axis cs:0.5,1.5) rectangle (axis cs:1.5,2.5);\fill [mycolor5] (axis cs:1.5,1.5) rectangle (axis cs:2.5,2.5);\fill [mycolor6] (axis cs:2.5,1.5) rectangle (axis cs:3.5,2.5);\fill [mycolor1] (axis cs:0.5,2.5) rectangle (axis cs:1.5,3.5);\fill [mycolor2] (axis cs:1.5,2.5) rectangle (axis cs:2.5,3.5);\fill [mycolor3] (axis cs:2.5,2.5) rectangle (axis cs:3.5,3.5);\end{axis}
...
```

Now:
```
...
	\fill [mycolor7] (axis cs:0.5,0.5) rectangle (axis cs:1.5,1.5);
	\fill [mycolor8] (axis cs:1.5,0.5) rectangle (axis cs:2.5,1.5);
	\fill [mycolor9] (axis cs:2.5,0.5) rectangle (axis cs:3.5,1.5);
	\fill [mycolor4] (axis cs:0.5,1.5) rectangle (axis cs:1.5,2.5);
	\fill [mycolor5] (axis cs:1.5,1.5) rectangle (axis cs:2.5,2.5);
	\fill [mycolor6] (axis cs:2.5,1.5) rectangle (axis cs:3.5,2.5);
	\fill [mycolor1] (axis cs:0.5,2.5) rectangle (axis cs:1.5,3.5);
	\fill [mycolor2] (axis cs:1.5,2.5) rectangle (axis cs:2.5,3.5);
	\fill [mycolor3] (axis cs:2.5,2.5) rectangle (axis cs:3.5,3.5);
...
```

It might be possible to create better output by using the capabilities `surf` or `patch` of pgfplots (not handled by this pull request).