Code cleanup to comply with project standards ...

Also, renamed a few tests and files

Make XLMarker indices congruent with the rest of ClosedXML ...

Previously XLMarker indices were zero-based. After this commit they will start
at one like the rest of column and row indices in the project.

Add support for other image formats ...

The format should be set on the Type property of the XLPicture object, it
should be set as a string, the class will then try to convert it to a valid
format. If it fails to do so, it will set the format to Jpeg.

Add image handling tests