Expect double rather than in for start/end/step

Following exception was thrown for start=0.0 when converting
an ome.tiff with modulo annotation back to ome.tiff:

```
Populating metadata
Exception in thread "main" java.lang.NumberFormatException: For input string: "0.0"
    at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
    at java.lang.Integer.parseInt(Integer.java:492)
    at java.lang.Integer.parseInt(Integer.java:527)
    at loci.formats.services.OMEXMLServiceImpl.getModuloAlong(OMEXMLServiceImpl.java:564)
    at loci.formats.services.OMEXMLServiceImpl.getModuloAlongT(OMEXMLServiceImpl.java:528)
    at loci.formats.in.OMETiffReader.initFile(OMETiffReader.java:834)
    at loci.formats.FormatReader.setId(FormatReader.java:1360)
    at loci.formats.ImageReader.setId(ImageReader.java:781)
    at loci.formats.tools.ImageConverter.testConvert(ImageConverter.java:337)
    at loci.formats.tools.ImageConverter.main(ImageConverter.java:672)
```
Skip modulo generation if one already exists

Previously, when trying:
```
./bfconvert -no-upgrade -overwrite FLIM-modulo-sample.ome.tiff /tmp/flim.ome.tiff
```

Two modulo annotation blocks appear in the output TIFF comment.
With this commit, later calls to addModulo() are silently ignored.