Inspection for "expand" invocations

[dievsky]

It would be nice to have an inspection for missing wildcards in expand expressions, e.g.

expand("{w1}_{w2}", w1="value")

should produce a warning because this statement can never succeed.

[iromeo]

• If expansion has allow_missing=True option, then missing wildcards are allowed.
• expand could accept a collection of elements, no just single path
• Also, user could pass python dictionary instead of w1="value" and such case rather hard to check
• Please be aware of escaping, e.g. in "{{w1}}_{{w2}}" is nothing to expand

A suggested example is rather trivial and users will rarely have bugs in such simple case. Much more useful is to allow this inspection to work with cases like: input: expand(rules.sort_tophat_bams.output, sample=SAMPLES, allow_missing=True) where:

rule sort_tophat_bams:
    output: temp("sorted/{genome}/tophat_{sample}_{genome}.bam")

It is important to cover different widely used scenarios here, e.g. when reference is to:

• whole section with 1 or more values, e.g. rules.foo.output or rules.foo.input, etc.
• selected output, e.g rules.foo.outptu[1] or rules.foo.input.key
• referenced section path could be wrapped into flag method, e.g. temp("sorted/{genome}/tophat_{sample}_{genome}.bam")
• referenced section path could be just reference to another rule