Add degrees and decimal minutes tick formatting mode

[DavidVadnais]

Rationale

Fixes: #944

Example code

import matplotlib.pyplot as plt
import cartopy.crs as ccrs
from cartopy.mpl.ticker import LongitudeFormatter, LatitudeFormatter

fig, axes = plt.subplots(1, 3, subplot_kw={'projection': ccrs.PlateCarree()},
                         figsize=(18, 5))

ax = axes[0]
ax.set_extent([-161, -154, 18, 23], crs=ccrs.PlateCarree())
ax.coastlines()
ax.set_xticks([-161, -159.51, -158, -156.3, -155, -154], crs=ccrs.PlateCarree())
ax.set_yticks([18, 19.51, 21, 22.3, 23], crs=ccrs.PlateCarree())
ax.xaxis.set_major_formatter(LongitudeFormatter(dms='dm'))
ax.yaxis.set_major_formatter(LatitudeFormatter(dms='dm'))
ax.set_title("dms='dm' (default format)")

ax = axes[1]
ax.set_extent([-161, -154, 18, 23], crs=ccrs.PlateCarree())
ax.coastlines()
ax.set_xticks([-161, -159.51, -158, -156.3, -155, -154], crs=ccrs.PlateCarree())
ax.set_yticks([18, 19.51, 21, 22.3, 23], crs=ccrs.PlateCarree())
ax.xaxis.set_major_formatter(LongitudeFormatter(dms='dm', minutes_number_format='.1f'))
ax.yaxis.set_major_formatter(LatitudeFormatter(dms='dm', minutes_number_format='.1f'))
ax.set_title("dms='dm' (minutes as .1f)")

ax = axes[2]
ax.set_extent([-161, -154, 18, 23], crs=ccrs.PlateCarree())
ax.coastlines()
ax.set_xticks([-161, -159.51, -158, -156.3, -155, -154], crs=ccrs.PlateCarree())
ax.set_yticks([18, 19.51, 21, 22.3, 23], crs=ccrs.PlateCarree())
ax.xaxis.set_major_formatter(LongitudeFormatter(dms=True))
ax.yaxis.set_major_formatter(LatitudeFormatter(dms=True))
ax.set_title("dms=True (degrees-minutes-seconds)")

plt.tight_layout()
plt.show()

Example output

Implications

I think the worst part of this PR is the change from dms: bool, optional to dms: bool or str, optional I'm interested in doing this differently but I'm not sure what you'd suggest? This implementation is backwards compatible which is good.

[greglucas]

There is a lot of extra code here, I think we should try to work this into what we already have instead if possible. We should also accept a dms="d", "dm", "dms" for any of the versions if someone wants degrees alone we don't want to have to write a bunch of new helpers.

I think this can be done within the formatting checks. We should always just use the value, deg, mn, sec = self._get_dms(abs(value)) Maybe we could add the checks into that function and return sec=0 if self._dms = "dm" which would then skip the seconds formatting. Similar to the minutes and degrees section.

[DavidVadnais]

Ive got the capability now but I haven't tackled the extra code fully. My next goal is to tighten this up.

[DavidVadnais]

I think this should just be deleted since it overwrites the default 5 for seemingly no reason.