Warn when variants in MAF file have wrong end position

[iskandr]

Some MAFs in the wild seem to have end positions which violate what's expected with base-1 inclusive coordinates, so warn about unexpected end positions instead of raising an exception. (fixes #105)

[tavinathanson]

Hmm. Do we really never want to ValueError here? What if we warn only in a specific off-by-one case, and ValueError otherwise?

[iskandr]

@tavinathanson I thought about trying to figure out what a safe category of errors is, but...what if our normalization of the ref/alt sequences makes the "safe" error range greater than 1?

[tavinathanson]

Thinking more about this: I feel like this still is saying more about the wonkiness of the MAF file than anything else, and the user probably should know that their MAF file is weird.

What about ValueError by default, unless the user loads the MAF as follows: variants = varcode.load_maf("tcga-ovarian-cancer-variants.maf", validate_maf=False)?

[tavinathanson]

Ah, but that would need to be annoyingly passed through from Topiary, given Kipp's use case.

Eh. Bleh. Feel free to just merge.