Incorrect usage of SVTYPE

[mbhall88]

The INFO tag SVTYPE is reserved for describing structural variants (see specs section 3). However, we use it to describe variant types instead. This could lead to problems with downstream VCF processing tools.

As an example, cyvcf2 uses the presence of that tag to indicate that a record is a SV. So, for a pandora VCF, every record would be considered an SV.

[leoisl]

Closed via #284 and #285