[API,FEATURE,TEST] Improved fitting of models to mass traces

- new method "getValue" to evaluate the model at a point
- more robust estimation of initial parameters for Gaussian model (caveat: fitting may use more steps in some cases)
- adapted tests
[FEATURE,PARAM] added checks for elution model validity; changed settings of OpenSWATH feature finding
[INTERNAL,TEST] increased accuracy of approximation for AUC calculation (for consistency), adapted tests
[FEATURE] PeakFileOptions extension

- extend PeakFileOptions by an maximal_data_pool_size_ parameter which
  specifies how many spectra should be read before being processed
[FEATURE] parallel spectra processing for mzML

- while reading, the spectra are decoded in parallel which allows for
  faster read times of the mzML data format
[FEATURE] parallel spectra processing for mzXML

- while reading, the spectra are decoded in parallel which allows for
  faster read times of the mzXML data format.
fix some forward declarations
[FIX,TEST] fix typo, add test
[FIX] fixes suggested by Chris

- correctly handle update of shared variable
- move PeakType out of loops
[FEATURE] Add parsing of TPP modifications to AASequence

- parse modifications in TPP format:

- allow parsing of modified peptides in TPP/SpectraST format with
  integer (PEPT[181]IDE) or accurate (PEPT[181.004]IDE) masses
- allow parsing of modified peptides in TPP/SpectraST format with
  integer (PEPT[+80]IDE) or accurate (PEPT[+79.96632]IDE) mass
  differences
[FIX] fix TraML Reader code
[FIX] stop parsing upon error
[BUILD] fix build system bug

- use OpenMSBuildSystem_externalLibs.cmake on top level to find correct
  include paths not only for OpenMS but also for OpenSwathAlgo and other
  libraries
- move the part of OpenMSBuildSystem_externalLibs.cmake specific to the
  openms-core (responsible for finding the actual library) to
  src/openms/cmake_findExternalLibs.cmake
- use OPENMS_HOST_DIRECTORY instead of PROJECT_SOURCE_DIR to determine
  correct location of contrib

fixes #699
[BUILD] rename file, fix comment
[TEST] updated test to reflect changes in GaussTraceFitter class
[INTERNAL] small simplification
[FIX] fixed checker warning (coding conventions)
[INTERNAL] added computeIntensityProfile() method to FFCentroided MassTraces

 * moved some code from header to source files
 * is faster then the previous map implementation currently used TraceFitters
[INTERNAL] utilize computeIntensityProfile in EGH- and GaussTraceFitter
[NOP,TEST] clean up MRMDecoy, add more tests