Bug fixes

This release mainly addresses a few minor bugs of the annoying but non-critical variety.

#54 The optional dependencies are now actually optional. This was an issue with how they were declared and used.
#55 An update to data.table changed behaviour of some type conversions and was causing fish4rodents() to crash. This is now fixed.
#56 A mysterious issue with some hdf5 library binaries prevents kallisto abundance.h5 files from being parsed by fish4rodents() using rhdf5. The problem remains unidentified, but as a workaround fish4rodents() can now alternatively parse the plaintext format of Kallisto output.

Enhancements and Changes

The *_p_mean field in the $Genes and $Transcripts tables is replaced with *_p_median instead, to better reflect the exponential/logarithmic way in which pvals are interpreted.
The *_p_stdev field in the $Genes and $Transcripts tables is removed as meaningless, for the same reason as above.
New fields *_Dprop_mean, *_Dprop_stdev, *_Dprop_max and *_Dprop_min are added to the $Transcripts table.
General house-keeping: Fixed various typos, tidied up code, improved code doc, updated vignette...