Start implementing seqfeature for genic structures

Create interface for gene transcript
Create tests for Exon and Intron
Implement transcript for coding transcripts
Create Region type and use embedding. Extract exon/intron in own files.
Add 5'UTR, CDS, 3'UTR in coding transcript
Add documentation for exon.
Remove redundant interface checks
New functions/fields for transcript

Use field strand instead of orientation.
Delete seq cause its not required yet.
Hide fields: utr5, cds and utr3.
Create non coding transcript type
Fix tests: switch from orientation to strand
add exoniclen to splicer - modify implementations
Restructure and rename files
New factory to create transcripts from GFF file
Merge branch 'seqfeature' into develop