Support an arbitrary number of VCZ files in create command

[tomwhite]

Currently it has to be exactly two, so must be called n-1 times if there are n VCZs that define the set of all variants.

[tomwhite]

Similarly, it should support creating a store with a single VCZ. To be consistent with the n>1 case this would create a new VCZ with zero samples that was otherwise the same as the supplied VCZ.

[tomwhite]

Edit: fixed in #105

Similarly, it should support creating a store with a single VCZ.

As a temporary workaround just specify the input VCZ twice:

vczstore create store.vcz vcz1 vcz1

This is inefficient as it compares vcz1 with itself, but it works.