Change to a CC0-based license. (#3831)

Unless otherwise noted, this project is in the public domain in the

United States.

It contains materials that originally came from the United States

Geological Survey, an agency of the United States Department of

Interior.  For more information on their copyright policies, see

https://www.usgs.gov/information-policies-and-instructions/copyrights-and-credits

It also contains materials from contributors that have waived their

copyright interest to the public domain.

Additionally, the authors waive copyright and related rights in the

work worldwide through the CC0 1.0 Universal public domain dedication.

CC0 1.0 Universal Summary

-------------------------

This is a human-readable summary of the [Legal Code (read the full

text)](https://creativecommons.org/publicdomain/zero/1.0/legalcode).

### No Copyright

The authors have associated their contributions to the ISIS project

with this deed, and have dedicated the work to the public domain

by waiving all of their rights to the work worldwide under copyright

law, including all related and neighboring rights, to the extent

allowed by law.

You can copy, modify, distribute and perform the work, even for

commercial purposes, all without asking permission.

### Other Information

In no way are the patent or trademark rights of any person affected

by CC0, nor are the rights that other persons may have in the work

or in how the work is used, such as publicity or privacy rights.

Unless expressly stated otherwise, the authors who have associated

the ISIS project with this deed make no warranties about the work,

and disclaim liability for all uses of the work, to the fullest

extent permitted by applicable law. When using or citing the work,

you should not imply endorsement by the authors.

This is free and unencumbered software released into the public domain.

Anyone is free to copy, modify, publish, use, compile, sell, or

distribute this software, either in source code form or as a compiled

binary, for any purpose, commercial or non-commercial, and by any

means.

In jurisdictions that recognize copyright laws, the author or authors

of this software dedicate any and all copyright interest in the

software to the public domain. We make this dedication for the benefit

of the public at large and to the detriment of our heirs and

successors. We intend this dedication to be an overt act of

relinquishment in perpetuity of all present and future rights to this

software under copyright law.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,

EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF

MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.

IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR

OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,

ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR

OTHER DEALINGS IN THE SOFTWARE.

For more information, please refer to <http://unlicense.org/>