- use correct way of specifying units
- avoid Hollerith constants

This is necessary to make to read member functions available which
previously where presented in sfftsfile.h

Code is reorganized in compilation units in smaller granularity.

The idea of providing inputoperators.h and outputoperators.h is to allow
to load the classes from modules in this library without loading all
input and output stream operators. Since sff::ts::File<C> provide member
functions for reading, this cannot be implemented strictly.

This is the first commit in a series of commits to fix a major bug:

Currently the source code fails to compile with g++ 7.3.1, although no
fatal errors were reported with previous compiler versions. Code
analysis reveals:
line 139 in sfftsfile.h calls the input operator
  datrw::idatstream& operator>>(datrw::idatstream& is, FileHeader& fh);
which is not available there. It is embarrassing that this went
unnoticed for such a long time. The input operator cannot be made
available there because of inappropriate separation of class declaration
and member function implementation. sfftsfile.h contains a declaration
required by inputoperators.h Hence sfftsfile.h amy not load inputoperators.h

Currently inputoperators.h appears like the main interface to libtsioxx,
loading all other header files subsequently. This appears inapprorpiate
as well.

The code must be reorganized in different compilation units, probably
introducing smaller granularity. This will change the API to the library
in terms of header files.

discard samples at beginning of trace in case filters result in transient
response to beginning of trace

The magnitude squared coherence appears to be the more common quantity in
literature.

The common measure of coherency in the literature appears to be magnitude
squared coherency. For this reason the library now implements this definition.
The previous scaling is provided as complex values of normalized cross-power
spectral density.