Add maven-antrun-plugin to download ERDDAP large file resources

[mwengren]

@ChrisPJohn This PR should be considered after #97.

Using maven-antrun-plugin, we can automate some processes to configure a local development environment for ERDDAP, including:

• download etopo1_ice_g_i2.bin file: currently, the target URL for the download here is the file location in the ERDDAP repo itself, but this can be moved outside of the code base in the future, such as a GitHub release artifact (as suggested by @ocefpaf at one point)
• download and unzip of the most recent erddapContent.zip file to a content subdirectory - with target URL configured via an added erddapcontent.download.version property in pom.xml
• make a local data directory (for running ERDDAP in place)

These commands can be run individually via:

mvn antrun:run@download-erddap-content
mvn antrun:run@download-etopo
mvn antrun:run@make-data-dir

They also should be executed automatically when a Maven build is run e.g. mvn package.

Note: I ran into an issue with the overwrite=false flag in the download-erddap-content task - docs seem to indicate this should prevent the unzip operation from overwriting destination files if they already exist, but it did not work when I tested. This comes into play if you modify the defaults in content/erddap/setup.xml and want to retain your changes after an ERDDAP version release - if you use this file for running ERDDAP in development mode, it will check to ensure defaults have been modified before fully starting. Not a deal breaker however.

[ChrisPJohn]

Thanks for sharing this. Since one of the goals of using maven to download large resources is to avoid having them in the GitHub repo, I do want to find an alternative place to host the resources before we merge this.

I also want to make sure we keep the developer set up simple. I want to think about what else is needed for developer environment set up (using maven to install dependencies and downloading the erddap.war file). Ideally all of the environment setup should be a single maven command if we're moving to using maven for that.