Add input file type auto-detect for count[graph|table]

[standage]

Add input file type auto-detect for count[graph|table]

[ctb]

Isn't there code in khmer that sniffs the file type? Much safer than relying on file extensions.

[ctb]

Looks like #71 is relevant.

[standage]

Ideally, it would be great to have khmer detect the file type and handle accordingly. Unless I'm reading the code incorrectly, though, sketches stored on disk only indicate the storage type (bit, byte, or nibble), and do not store whether they are a *table or *graph data structure. So additional input is needed from the user anyway, and that might as well come in the form of a filename, with all the challenges that entails.

All of this is ripe for a fresh look once dib-lab/khmer#1595 is merged. @camillescott may well have already made some changes to the relevant code, but regardless we need to consider how we are going to support a growing list of hash functions (2bit, MurmurHash, rolling hash or hashes, more) and pre-processing options (consume with or without k-mer banding, consume with or without a mask, consume with or without [^ACGT] preprocessing) in a maintainable way.