Compilation error fixes

- In C you're not allowed to define variables in the middle
  of the block. This was violated in curve_fitting.c by
  calling ceres_init() in the beginning of main() and declaring
  variables later.

- Also ifdef-ed suitesparse stuff in covariance estimation module.
  This solves compilation error when you don't have suitesparse
  compiled/installed.

Change-Id: I22b543c09ea01f55e127079daade99a0b781f789