fdo#40314 Free value of intercept for trend line

Intercept value field has been changed to FormattedField
to support scientific notation and more than 4 digits
after decimal point

Change-Id: Idb2d27862ddabcdc96a036d93f1f3498cc6327df
Reviewed-on: https://gerrit.libreoffice.org/6904
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>
Tested-by: Tomaž Vajngerl <quikee@gmail.com>
Fix detection of force intercept in XLS 97/2000/XP format.

NaN value indicates if the force intercept is enabled or not.

Change-Id: I51d33a6e04a4b5832f241e7ae7ee6d2fbb32ea9b