Design-Change

[Lukas113]

In order to not to have to commit changes to a different branch each time for a new build, the feedstock should be changed to the following setup:

• Provide source (git tag or commit hash as string) and conda build target (Version incl. dev as string) of the conda-build action with default values according to the latest package and version to build. Dev builds are then handled by just provide the PEP440 dev-version as string.
• Provide a set of conda-sub-dep targets (versions as strings) with default values according to the latest package and version to build. Dev deps are then just provided using the PEP440 dev-version as string.
• Adapt README to changes. In addition, build-trees may have to go. Otherwise it may be too complicated to support different sub-dep versions if necessary.
• Check wheel before building it in case it already exists. E.g. using wget?
• Just range, and don't fix larger sub-deps like numpy or astropy for each sub-dep. Otherwise it may result in unresolvable environments.