Systemrdl-compiler

**New Features**
- Add `is_msb0_order` marker to `Reg` component
- Add array index zero/clear methods to Node
- Add `raw_absolute_address` to Node

**Bug Fixes**
- Fix enum entry ordering

**New Features**
- Expose markdown processor class to user when generating HTML descriptions
- Semantic checks for virtual registers
- Additional semantic checks for properties: `dontcompare`, `donttest`, `onread`, `onwrite`, `singlepulse`, `we`, `wel`, `encode`
- Add debug and info message severities. (22)

**Bug Fixes**
- Fix derivation of default `memwidth`
- Fix a few missed scenarios in `FieldNode.implements_storage` meta-property

**New Features**
- Initial API support for register model importers (19)
- Add new optional `SPARSE_REG_STRIDE` warning.
- Bring forward several component variables into Node for convenience.

**Bug Fixes**
- Fix exception when formatting empty desc string (18)
- Fix missing cross-implications between woset, woclr, onwrite property values.
- Fix issue with error message context within included files (21)

**Bug Fixes**
- Fix exception when printing error message (17)

**New Features**
- Add new recursive ``Node.descendants()`` iterator
- Back-port support for Python 3.4 (14, 15, 16)
- Add ability to identify component's declaration scope via ``Component.parent_scope`` and ``Component.get_scope_path()``
- Rework warnings subsystem

**New Features**
- Add ability to override top-level component's parameters via `elaborate()`
- Implement internal/external instance types (11)

**Bug Fixes**
- Fix nested parameter propagation bug (12)