work towards enforcing availability of SHA256 checksums for all sources/patches

[boegel]

This PR makes a couple of changes:

• also support sha256 and sha512 as checksum types
• auto-detect MD5 vs SHA256 checksums when no checksum type is specified (based on length of provided checksum)
• report both MD5 and SHA256 checksums in log for all sources/patches (incl. extensions)
• add --enforce-checksums option to enforce presence of checksums for all sources/patches

It's a first step towards having SHA256 checksums for all sources/patches in all centrally provided easyconfigs.

[ocaisa]

We could also add the checksums automatically to the archived easyconfig

[boegel]

@ocaisa Sure, but I think that should be done in a separate PR, since that's a bit fiddly (you need to inject checksums =, preferably under sources = ...)

Also, there's a bit of chicken-egg situation there, I think, especially if we want to add SHA256 checksums automatically, i.e. they could only be consumed with an EasyBuild version that includes support for (auto-detection of) SHA256 checksums...

[vanzod]

This PR seems good to me. Nice work

[boegel]

Thanks for the review @wpoely86 and @vanzod!