Disallow construction of k-mers from integers

[jakobnissen]

Right now, you can do

julia> DNACodon(11)
DNA 3-mer:
AGT

I don't think you should be able to do that. The fact that the codon is internally represented as UInt(11) is an implementation detail, and should not be part of the API.

I suggest removing it.

Edit: Changed the following:

• Removed conversion to/from integers/kmers. This is dangerous since conversion can happen implicitly
• Construction of kmers from integers is still permitted, but now checks for overflow, e.g. DNAMer{2}(0xfffff) throws a DomainError
• To avoid the check, use the function masked, which instead masks the noncoding bits.

[jakobnissen]

See edited original comment