Add parameter SolidColor to STL writer and reader

STL files are very basic. However, they are useful as an exchange format for many vendors support them. While there is no single convention on how to store color information in STL files, some vendors have chosen to write "COLOR=RGBA" in the (binary) header.

The changes also allow the user to access the full header after reading the STL file.

Added simple test case to show that color is written and can be read back.