improve maps downloading in `build-coatjava.sh`

[c-dilks]

• --clean should not be downloading maps
• add option --redownload-maps to force a download of maps (by removing --timestamping argument from wget)

[c-dilks]

alternatively, use Maven to handle this...

[c-dilks]

add option --redownload-maps to force a download of maps (by removing --timestamping argument from wget)

Decided to just print a note, to avoid adding yet another control option to build-coatjava.sh...