GEMMA should give hints if matrix properties are invalid, such as discussed in #45 (comment). E.g.

1. Fail if K has negative eigen values
2. Fail if K is not symmetric
3. Fail if K is not positive definite
4. Warn in eigen values are very small
5. Warn if K is ill conditioned
6. Warn on related pairs
7. Warn on MAF problems

@xiangzhou @pcarbo anything else we can think of?

Failures and warnings should be reported in the log file. These checks can be disabled with the --no-checks switch (i.e., dangerous mode but faster).