add REUSE.software license information

[fneum]

Closes #65.

Added FSFE REUSE compliant license information. Documentation now licensed under CC-BY-4.0

[euronion]

quick x-check for completness:

pypsa-eur>reuse lint
# SUMMARY

* Bad licenses:
* Deprecated licenses:
* Licenses without file extension:
* Missing licenses:
* Unused licenses:
* Used licenses: CC-BY-4.0, CC0-1.0, GPL-3.0-or-later
* Read errors: 0
* Files with copyright information: 152 / 152
* Files with license information: 152 / 152

Congratulations! Your project is compliant with version 3.0 of the REUSE Specification :-)

[fneum]

CI failure on Windows only due to separate issue. Merge anyway with "[skip travis]". Opened dedicated issue #161.

Done. Your build exited with 1.

Traceback (most recent call last):

  File "C:\miniconda\envs\pypsa-eur\lib\site-packages\snakemake\__init__.py", line 699, in snakemake

    keepincomplete=keep_incomplete,

  File "C:\miniconda\envs\pypsa-eur\lib\site-packages\snakemake\workflow.py", line 632, in execute

    or delete_temp_output,

  File "C:\miniconda\envs\pypsa-eur\lib\site-packages\snakemake\persistence.py", line 72, in __init__

    os.makedirs(d, exist_ok=True)

  File "C:\miniconda\envs\pypsa-eur\lib\os.py", line 223, in makedirs

    mkdir(name, mode)

NotADirectoryError: [WinError 267] The directory name is invalid: 'C:\\Users\\travis\\build\\PyPSA\\pypsa-eur\\.snakemake\\aux'