FHIR feature updates (#285)

Partially addresses:
- timsbiomed/issues#85

## Overview
- Minor bug fixes
- Dev updates (comments, etc)
- Added test(s)

## Updates
`7b91d57da9a5143c5ce8335897ad2542424797c6`
```
    - Add: _test_to_fhir_json()
```

`157ae226921891feeaa38b4329c98b762b2b8727`
```
    - Refactored 3 JSON writer functions to a state that they were probably intended to be in: 1 json function with branching logic depending on 'serialization' param.
```

`0ab8625887a1d57adeadd5ad6d3c47dd08524839`
```
    - Update: Minor codestyle updates, comments, todo's
    - Delete: Unused schema/fhir_json.csv. Mappings will continue to be done via pure Python
    - Bugfix: extension.ValueString -> extension.valueString
    - Delete: Comments about spec field mappings
```

`0dd0772dc7bc4a263fa8e8f8e5c02c6f6c3264bf`
```
    - Added: schema/fhir_json.csv: For automation of conversion by utilizing CSV export from curated GoogleSheet.
    - Update: Comments: Added state of mappings between SSSOM and FHIR ConceptMap
```