Add ClinicalTrials.gov source

[cthoyt]

This PR adds an initial ClinicalTrials.gov source. Currently, it gets information from:

• ID / Name
• Classification by study type / allocation (interventional/observational/expanded access and randomized/nonrandomized)
• References
• Conditions (when available from the derived section)
• Interventions (when available from the derived section)

There are lots more things to do to parse content out of here, but PyOBO typically is not a full data science workflow that also incorporates information extraction, so some things will get left out for now.

The OBO, OFN, and OWL artifacts from build are available at https://github.com/biopragmatics/obo-db-ingest/tree/main/export/clinicaltrials. Here's some example stanzas from the OBO output:

[Instance]
id: clinicaltrials:NCT00000102
name: Congenital Adrenal Hyperplasia\: Calcium Channels as Therapeutic Targets
property_value: clinicaltrials:has_intervention mesh:D009543 ! has intervention Nifedipine
property_value: clinicaltrials:investigates_condition mesh:D000308 ! investigates condition Adrenocortical Hyperfunction
property_value: clinicaltrials:investigates_condition mesh:D000312 ! investigates condition Adrenal Hyperplasia, Congenital
property_value: clinicaltrials:investigates_condition mesh:D006965 ! investigates condition Hyperplasia
property_value: clinicaltrials:investigates_condition mesh:D047808 ! investigates condition Adrenogenital Syndrome
instance_of: interventional-clinical-trial

[Instance]
id: clinicaltrials:NCT00000104
name: Does Lead Burden Alter Neuropsychological Development?
property_value: clinicaltrials:investigates_condition mesh:D007855 ! investigates condition Lead Poisoning
property_value: clinicaltrials:investigates_condition mesh:D011041 ! investigates condition Poisoning
instance_of: observational-clinical-trial

[Instance]
id: clinicaltrials:NCT00000106
name: 41.8 Degree Centigrade Whole Body Hyperthermia for the Treatment of Rheumatoid Diseases
property_value: clinicaltrials:investigates_condition mesh:D003095 ! investigates condition Collagen Diseases
property_value: clinicaltrials:investigates_condition mesh:D012216 ! investigates condition Rheumatic Diseases
instance_of: randomized-interventional-clinical-trial

[Instance]
id: clinicaltrials:NCT00000250
name: Cold Water Immersion Modulates Reinforcing Effects of Nitrous Oxide - 2
property_value: clinicaltrials:has_intervention mesh:D009609 ! has intervention Nitrous Oxide
property_value: clinicaltrials:investigates_condition mesh:D009293 ! investigates condition Opioid-Related Disorders
property_value: clinicaltrials:investigates_condition mesh:D019966 ! investigates condition Substance-Related Disorders
instance_of: non-randomized-interventional-clinical-trial

[Instance]
id: clinicaltrials:NCT00040625
name: ALIMTA \(Pemetrexed\) Alone or in Combination With Cisplatin for Patients With Malignant Mesothelioma.
property_value: clinicaltrials:has_intervention mesh:D000068437 ! has intervention Pemetrexed
property_value: clinicaltrials:investigates_condition mesh:D000086002 ! investigates condition Mesothelioma, Malignant
property_value: clinicaltrials:investigates_condition mesh:D008654 ! investigates condition Mesothelioma
instance_of: expanded-access-study

Note that it uses ad-hoc terms and relations, this is being discussed in obi-ontology/obi#1831.

It would also be nice to have a way of annotating phases, which have the following statistics over the database:

Phase	Count
NA	196036
	122727
PHASE2	59412
PHASE1	44195
PHASE3	39160
PHASE4	33129
PHASE1,PHASE2	15219
PHASE2,PHASE3	6982
EARLY_PHASE1	5434