Update instructions on downloadIsisData and conda install in README (#5159)

* Update downloadIsisData in README

The ISIS SPICE Web Service section still uses rsync, updated to downloadIsisData, and added some instructions for using downloadIsisData

* Add channel_priority setting to README

channel_priority=strict would cause package conflicts

* Update .zenodo.json

* Update CHANGELOG.md

* Update CHANGELOG.md

* Update Jenkinsfile setting channel_priority