added sequence, coding sequence, and UTR properties to Transcript objects



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```
time ts = [Transcript(transcript_id, ensembl.db, ensembl.reference) for transcript_id in transcript_ids]
```
```
CPU times: user 15.2 s, sys: 5.45 s, total: 20.6 s
Wall time: 28.6 s
```
vs. 
```
time ts = [Transcript(transcript_id, db, reference) for transcript_id in transcript_ids]
```
```
CPU times: user 13.3 s, sys: 3.91 s, total: 17.2 s
Wall time: 19.8 s
```