Changelog

Additions

Added sequence.ProteinSequence.get_molecular_weight() method
Added application.sra subpackage as interface to NCBI SRA tools
- FastqDumpApp is used for fetching FASTQ files from the NCBI SRA
Added iter_read() static method to sequence.io.fasta.FastaFile
and sequence.io.fasta.FastqFile
- This method is used to parse header-sequence-pairs from FASTA/FASTQ
  files without the necessity to keep the entire file in memory.
set_sequence and set_sequences in sequence.io.fasta
and sequence.io.fasta support writing RNA sequences with the new
as_rna parameter

Fixed missing whitespace at the end of _loop category labels in
PDBx/mmCIF files (#224)
Fixed inconsistent handling of model IDs over different file formats
for structures where the first model ID is greater than 1 (#227)
Removed warning in structure.density()
sequence.io.fastq.get_sequence() and sequence.io.fastq.get_sequences()
properly handle RNA and ambiguous sequences now
Fixed start parameter in structure.renumber_atom_ids and
structure.renumber_res_ids
Updated fetch URL for FASTA files in database.rcsb.fetch()