refactor mesh & MCA data parsing code

[prjemian]

per #72, the code that converts the raw SPEC data into a 2-D grid resides in writer.Writer.mesh()

this code will be used in more than one place now (writer, specplot) so move it into an independent location

same for writer.Writer.mca_spectra()

---

from #151 : as noted in the source code:

spec2nexus/src/spec2nexus/writer.py

Lines 238 to 240 in 304b0fe

	def mesh(self, nxdata, scan):
	"""*internal*: data parser for 2-D mesh and hklmesh"""
	# TODO: refactor to use NeXus data model: signal, axes, data

added:
resolve TODO items in these files:

docs/source/nexus_layout/mca.rst:TODO: write
docs/source/nexus_layout/mca.rst:TODO: show with one MCA
docs/source/nexus_layout/mca.rst:TODO: show with CALIB

[prjemian]

CAUTION: Do this in steps, with unit testing on the spec2nexus.writer.Writer() code to ensure no consequences on the user community

[prjemian]

This work was undone in commit 2c9b541.
Can we bring those changes back?

[prjemian]

This is a refactoring of the code that potentially breaks the existing API (by changing the imports). Move to milestone for next major version number.