Rename several metadata fields

[dfornika]

As requested by @apetkau and @jrober84, rename metadata field names as follows:

current_field_name	new_field_name	note	field_value_example
average coverage status	qc_status_read_depth_fixed		PASSED
average coverage value	mean_read_depth_fixed		286.8
basecount	total_sequenced_bases		509081917
checkm status	qc_status_contamination		PASSED
checkm value	checkm_value		0.66
count_loci_found	locidex_count_loci_found		4129
count_loci_missing	locidex_count_loci_missing		13188
gc (%)	gc		50.43
length status	qc_status_assembly_genome_size		PASSED
length value	assembly_length		4785830
mean sequence length forward			101
	mean sequence length reverse	add the reverse read field as well	101
	qc_status_read_length	add the read length status (> 100 bp)
n50 status			PASSED
n50 value			101963
nr contigs status	qc_status_assembly_num_contigs		PASSED
nr contigs value	assembly_num_contigs		73
qc status	qc_status_overall		PASSED
qc summary	qc_summary		PASSED Species ID: Escherichia coli; Passed Tests: 6/6; Organism QC Criteria: Escherichia coli
raw average quality status	qc_status_read_qual		PASSED
raw average quality value			34.49
total_loci	locidex_db_loci_total_count		17317
	qc_status_num_loci	add the qc message with num loci >= min per scheme