Add support for markers that don't have a diameter

Allow markers to have no diameters (#233)

Markers may have only `(X Y Z)` specified instead of the more common `(X Y Z D)`. In this case, diameters are set to 0.

Also:
Strings in the middle of the list of points are now skipped.

ℹ️ The example file added in this commit is from C280999A-I4.asc

The following illustrates both use cases:

```lisp
( (Color Red)
  (   -0.97  -141.17    84.77)
  "<--4 boutons on a PC soma"
)  ;  End of text
```