This seems to fix visibility issues with EPICS base versions above 3.14.

The reason is that unfortunately sphinx does not create download links
for files that do not exist in the source tree.

* The search for a matching interval in a table of break-points was optimized.
  The lookup function now looks first to the previous interval, then the
  intervals below and above the previous one. Only if these intervals don't
  match it performs a binary search across the while break point table.
* If the functions csm_x, csm_y or csm_z are called with the same values as the
  last time, they do not perform any calculation but return the previous value
  from their internal cache.

lookup_1d_functiontable now handles the case of a table with a single
point (xp,yp) correctly. It then returns the associated yp for any value of
x. Before this change, it returned NAN for x!=xp and yp for x==xp.

Additionally, the functions lookup_1d_functiontable and
lookup_2d_functiontable now return NAN (not a number) in case the table
doesn't contain any values.

This is consistent with how the records in base work and avoids
misleading error messages at startup time when CA input links
have not yet connected.

Also added new fields IAOM and IAOV to facilitate this.

Also convert cvtRecord.html to cvtRecord.rst, and
integrate the csm doxygen generated API docs.