allow "xcube serve" to serve cubes given via CLI paths

[forman]

This is a prerequisite for other QC and performance measurement tools related to solving #52 (PR will follow soon).

@AliceBalfanz please merge after review.

[AliceBalfanz]

One test still failing, please fix before merging.

[forman]

@AliceBalfanz once again, please!

[codecov-io]

Codecov Report

Merging #68 into master will decrease coverage by 0.1%.
The diff coverage is 59.72%.

@@            Coverage Diff            @@
##           master     #68      +/-   ##
=========================================
- Coverage    84.6%   84.5%   -0.11%     
=========================================
  Files          70      70              
  Lines        5976    6021      +45     
=========================================
+ Hits         5056    5088      +32     
- Misses        920     933      +13

Impacted Files	Coverage Δ
xcube/webapi/defaults.py	100% <ø> (ø)	⬆️
xcube/cli/serve.py	67.64% <42.85%> (+4.31%)	⬆️
xcube/webapi/context.py	78.34% <52.17%> (-0.95%)	⬇️
xcube/webapi/service.py	60.36% <71.42%> (+2.38%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 98d1878...b77c4be. Read the comment docs.

[AliceBalfanz]

Great :)