Kalman: only initialise param vars one time

[krichardsson]

The parameters used in kalman core were extracted into a struct in #869.
The variables used for the parameters are reset to default values when the kalman estimator is reset, and this causes two problems:

1. Parameter values that were set earlier are lost, for instance the starting point used in the Position commander (see Position estimate can no longer be initialized to non-zero value #1014)
2. The contract for parameters is that the underlying variable should not be changed directly by the firmware code, as the client can not detect the change. Re-initializing the parameters from time to time breaks this contract.

In this PR the initialization of the parameter data is moved from estimatorKalmanInit() to estimatorKalmanTaskInit() to make sure it is only done one time.

Fixes #1014

[tobbeanton]

Good change.

[krichardsson]

@whoenig We decided to merge this even though you did not review it, please let us know if this causes any issues for the python bindings