Add MSI measure

[mbenary]

Is your feature request related to a problem? Please describe.
MSI status is a relevant feature to describe tumor stability and is relevant for clinical decision-making. Currently, snappy provides MANTIS, but there are other options available.

Describe the solution you'd like
First, the wrapper for MANTIS needs to be tested. The output needs to be available for cbioportal-export and that should be homogenous across different tools.

Describe alternatives you've considered
Provide wrapper for other MSI callers, which allow tumor-only samples, such as MSIsensor2 which is used by TCGA

Additional context
None.