fix:save scaled value, perform initial get

[nulinspiratie]

Minor but important Parameter fixes I ran into after merging the latest PR.
These are related to ramping and scaling of values, and also incorporating raw_value.

Changes proposed in this pull request:

• Perform Parameter.get if get_latest returns None and a step is used. Without this, it will jump to the target value during the first .set if not .get is called first
• scale parameter value during ramping
• Remove raw_value from meta_attrs, add it to self._latest.
  This is done for the following reasons:
  • The self.get function (previously in ManualParameter) is no longer equal to self.get_latest. Instead, it will directly get the value from self._latest ('value' if parameter doesn't have scale, 'raw_value' otherwise). This is because get_latest always returns the scaled value, while if the parameter has a scale, we want the get function to return the raw value
  • the raw value will now also be popped if snapshot_value=False

@jenshnielsen @WilliamHPNielsen

[WilliamHPNielsen]

This has been superseded by #788.